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    How does a hard drive work? Explain that Stuff

    Hard drives

    by Chris Woodford. Last updated: August 24, 2015.

    M ost people are amazed when they discover they can store hundreds of CDs worth of music on an iPod digital music player no bigger than a pack of cards. An iPod (one of the older ones, anyway) is not much more than a hard drive. an incredibly efficient computer memory device that uses simple magnetism to store vast amounts of information. Hard drives were invented over 50 years ago and have been used in personal computers since the mid-1980s. The microprocessor in your computer is the bit that does all the "thinking" and calculating—but it's the hard drive that gives your computer its prodigious memory and lets you store digital photos. music files, and text documents. How does it work? Let's take a closer look!

    Photo: A typical 30GB (gigabyte) hard drive from a laptop computer.

    How to store information with magnetism

    The science of magnetism is complex. But if you've ever fooled around with a magnet and some nails, you'll know that the technology—the science in action—is quite simple. Iron nails start off unmagnetized but, if you rub a magnet back and forth over them, you can make them magnetic so they stick to one another. Magnetism has some simple, practical uses. For example, junkyards use electromagnets (huge magnets that can be switched on and off with electricity ) to pick up and move around piles of metal scrap.

    Magnetism has another very important use. Suppose you need to leave a message for a friend and all you have is a magnet and an unmagnetized iron nail. Suppose the message is a very simple one: either you will see your friend later that day or not. You could arrange with your friend that you will drop a nail through their letterbox. If the nail is magnetized, it means you will see them later; if the nail is unmagnetized, you won't. Your friend gets in from school and finds a nail on the doormat. They take it to the kitchen table and try to pick up a paperclip. If the clip attaches to the magnet, it must be magnetized—and it must mean you plan to see them later. It's a pretty weird way to leave a message for someone, but it illustrates something very important: magnetism can be used to store information.

    Photo: Magnets—the technology behind hard drives really is this simple!

    If your computer has a 20 gigabyte (GB) hard drive, or you have a 20 GB iPod or MP3 player, it's a bit like a box containing 160 thousand million microscopically small iron nails, each of which can store one tiny piece of information called a bit. A bit is a binary digit—either a number zero or a number one. In computers, numbers are stored not as decimal (base-10) but as patterns of binary digits instead. For example, the decimal number 382 is stored as the binary number 101111110. Letters and other characters can also be stored as binary numbers. Thus, computers store a capital letter A as the decimal number 65 or the binary number 1000001. Suppose you want to store the number 1000001 in your computer in that big box of iron nails. You need to find a row of seven unused nails. You magnetize the first one (to store a 1), leave the next five demagnetized (to store five zeros), and magnetize the last one (to store a 1).

    How a hard drive works

    In your computer's hard drive, there aren't really any iron nails. There's just a large shiny, circular "plate" of magnetic material called a platter. divided into billions of tiny areas. Each one of those areas can be independently magnetized (to store a 1) or demagnetized (to store a 0). Magnetism is used in computer storage because it goes on storing information even when the power is switched off. If you magnetize a nail, it stays magnetized until you demagnetize it. In much the same way, the computerized information (or data) stored in your PC hard drive or iPod stays there even when you switch the power off.

    What are the parts in a hard drive?

    A hard drive has only a few basic parts. There are one or more shiny silver platters where information is stored magnetically, there's an arm mechanism that moves a tiny magnet called a read-write head back and forth over the platters to record or store information, and there's an electronic circuit to control everything and act as a link between the hard drive and the rest of your computer.

    After a hard-drive crash last year, I was left with an old drive that no longer worked. I took a peek inside, and here's what I found.

    1. Actuator that moves the read-write arm. In older hard drives, the actuators were stepper motors. In most modern hard drives, voice coils are used instead. As their name suggests, these are simple electromagnets, working rather like the moving coils that make sounds in loudspeakers. They position the read-write arm more quickly, precisely, and reliably than stepper motors and are less sensitive to problems such as temperature variations.
    2. Read-write arm swings read-write head back and forth across platter.
    3. Central spindle allows platter to rotate at high speed.
    4. Magnetic platter stores information in binary form.
    5. Plug connections link hard drive to circuit board in personal computer.
    6. Read-write head is a tiny magnet on the end of the read-write arm.
    7. Circuit board on underside controls the flow of data to and from the platter.
    8. Flexible connector carries data from circuit board to read-write head and platter.
    9. Small spindle allows read-write arm to swing across platter.

    Photo: Little and large: Here's the 30GB laptop hard-drive (shown in the other photos on this page) next to a 20GB PCMCIA hard drive from an iPod. The two drives look strikingly similar and work exactly the same way (both are made by Toshiba), but the iPod drive is even more of a miracle of miniaturization!

    The platters are the most important parts of a hard drive. As the name suggests, they are disks made from a hard material such as glass or aluminum. which is coated with a thin layer of metal that can be magnetized or demagnetized. A small hard drive typically has only one platter, but each side of it has a magnetic coating. Bigger drives have a series of platters stacked on a central spindle, with a small gap in between them. The platters rotate at up to 10,000 revolutions per minute (rpm) so the read-write heads can access any part of them.

    There are two read-write heads for each platter, one to read the top surface and one to read the bottom, so a hard drive that has five platters (say) would need ten separate read-write heads. The read-write heads are mounted on an electrically controlled arm that moves from the center of the drive to the outer edge and back again. To reduce wear and tear, they don't actually touch the platter: there's a layer of fluid or air between the head and the platter surface.

    Reading and writing data

    The most important thing about memory is not being able to store information but being able to find it later. Imagine storing a magnetized iron nail in a pile of 1.6 million million identical nails and you'll have some idea how much trouble your computer would get into if it didn't use a very methodical way of filing its information.

    When your computer stores data on its hard drive, it doesn't just throw magnetized nails into a box, all jumbled up together. The data is stored in a very orderly pattern on each platter. Bits of data are arranged in concentric, circular paths called tracks. Each track is broken up into smaller areas called sectors. Part of the hard drive stores a map of sectors that have already been used up and others that are still free. (In Windows, this map is called the File Allocation Table or FAT .) When the computer wants to store new information, it takes a look at the map to find some free sectors. Then it instructs the read-write head to move across the platter to exactly the right location and store the data there. To read information, the same process runs in reverse.

    With so much information stored in such a tiny amount of space, a hard drive is a remarkable piece of engineering. That brings benefits (such as being able to store 500 CDs on your iPod)—but drawbacks too. One of them is that hard drives can go wrong if they get dirt or dust inside them. A tiny piece of dust can make the read-write head bounce up and down, crashing into the platter and damaging its magnetic material. This is known as a disk crash (or head crash ) and it can (though it doesn't always) cause the loss of all the information on a hard drive. A disk crash usually occurs out of the blue, without any warning. That's why you should always keep backup copies of your important documents and files, either on another hard drive, on a compact disc (CD) or DVD. or on a flash memory stick.

    Photo: The read-write head on a hard-drive. Left: The actuator arm swings the head back and forth so it's in the right position on the drive. Right: Only the tiny extreme end part of the hard drive actually reads from and writes to the platter. Bear in mind that half of what you're seeing in the right photo is a reflection in the shiny hard drive surface!

    Who invented the hard drive?

    Like many innovations in 20th-century computing, hard drives were invented at IBM as a way to give computers a rapidly accessible "random-access" memory. The trouble with other computer memory devices, like punched cards and reels of magnetic tape, is that they can only be accessed serially (in order, from beginning to end), so if the bit of data you want to retrieve is somewhere in the middle of your tape, you have to read or scan through the entire thing, fairly slowly, to find the thing you want. Everything is much faster with a hard drive, which can move its read-write head very quickly from one part of the disk to another; any part of the disk can be accessed as easily as any other part. The first hard drive was developed by IBM's Reynold B. Johnson and announced on September 4, 1956 as the IBM 350 Disk Storage Unit. IBM engineers also pioneered floppy disks, which were removable magnetic disks packed in robust plastic cases (originally 20cm or 8in in diameter and wrapped in flexible plastic sleeves; later 133mm or 5.25in in diameter and packed in tough plastic cases). Developed by IBM's Warren Dalziel in 1967 and first sold in 1971, they became hugely popular in microcomputers (the forerunners of PCs) in the late 1970s and early 1980s.

    Photo: This 3.5mm floppy disk has a magnetic plastic disk inside that spins round in much the same way as the platter in a hard drive. Although commonplace in the 1980s, floppies are now obsolete. With a storage capacity of only 1.44MB, they've been completely superseded by USB flash "drives" that offer hundreds or thousands of times more memory in a tiny plastic stick a fraction the size.

    What is hard drive?

    Hard drive

    A hard disk drive (sometimes abbreviated as Hard drive. HD. or HDD ) is a non-volatile memory hardware device that permanently stores and retrieves information. There are many variations, but their sizes are generally 3.5" and 2.5" for desktop and laptop computers respectively. A hard drive consists of one or more platters to which data is written using a magnetic head, all inside of an air-sealed casing. Internal hard disks reside in a drive bay, connect to the motherboard using an ATA. SCSI. or SATA cable, and are powered by a connection to the PSU (power supply unit).

    A hard drive can be used to store just about any type of data, including pictures, music, videos, and text documents. Computers have a hard drive and use it to store files for the operating system and software that run on the computer, as well as files created or downloaded to the computer by a user.

    Hard drive components

    As can be seen in the picture above, the desktop hard drive consists of the following components: the head actuator. read/write actuator arm. read/write head. spindle. and platter. On the back of a hard drive is a circuit board called the disk controller .

    Tip: New users often confuse memory (RAM) with disk drive space. See our memory definition for a comparison between memory and storage.

    Note: The above picture is an example of a traditional hard drive and not an SSD .

    How is data read and stored on a hard drive?

    Data sent to and read from the hard drive is interpreted by the disk controller. which tells the hard drive what to do and how to move the components within the drive. When the operating system needs to read or write information, it examines the hard drive's File Allocation Table (FAT) to determine file location and available write areas. Once they have been determined, the disk controller instructs the actuator to move the read/write arm and align the read/write head. Because files are often scattered throughout the platter, the head needs to move to different locations to access all information.

    All information stored on a traditional hard drive, like the above example, is done magnetically. After completing the above steps, if the computer needs to read information from the hard drive, it would read the magnetic polarities on the platter. One side of the magnetic polarity is 0, and the other is 1. Reading this as binary data, the computer can understand what the data is on the platter. For the computer to write information to the platter, the read/write head aligns the magnetic polarities, writing 0's and 1's that can be read later.

    External and Internal hard drives

    Although most hard drives are internal, there are also stand-alone devices called external hard drives. which can backup data on computers and expand the available disk space. External drives are often stored in an enclosure that helps protect the drive and allows it to interface with the computer, usually over USB or eSATA. A great example of an external backup device that supports multiple hard drives is the Drobo .

    External hard drives come in many shapes and sizes. Some are large, about the size of a book, while others are about the size of a cell phone. External hard drives can be very useful since they usually offer more space than a jump drive and are still portable. The picture to the right is an example of a laptop hard disk drive enclosure from Adaptec. The user may install a laptop hard drive of any storage capacity into the enclosure and connect it via USB port to the computer.

    HDD being replaced by SSD

    Solid State Drives (SSDs) have started to replace hard disk drives (HDDs) because of the distinct performance advantages they have over HDD, including faster access times and lower latency. While SSDs is becoming more and more popular, HDDs continue to be used in many desktop computers largely due to the value per dollar that HDDs offer over SSDs. However, more and more laptops are beginning to utilize SSD over HDD, helping to improve the reliability and stability of laptops.

    History of the hard drive

    The first hard drive was introduced to the market by IBM on September 13, 1956. The hard drive was first used in the RAMAC 305 system, with a storage capacity of 5 MB and a cost of about $50,000 ($10,000 per megavbyte). The hard drive was built-in to the computer and was not removable.

    In 1963. IBM developed the first removable hard drive, having a 2.6 MB storage capacity.

    The first hard drive to have a storage capacity of one gigabyte was also developed by IBM in 1980. It weighed 550 pounds and cost $40,000.

    1983 marked the introduction of the first 3.5 inch size hard drive, developed by Rodime. It had a storage capacity of 10 MB.

    Seagate was the first company to introduce a 7200 RPM hard drive in 1992. Seagate also introduced the first 10,000 RPM hard drive in 1996 and the first 15,000 RPM hard drive in 2000 .

    The first solid-state drive (SSD) as we know them today was developed by SanDisk Corporation in 1991. with a storage capacity of 20 MB. However, this was not a flash-based SSD, which were introduced later in 1995 by M-Systems. These drives did not require a battery to keep data stored on the memory chips, making them a non-volatile storage medium.

    Other help and related links

    What is Hard Disk Drive? Webopedia

    HDD - hard disk drive Related Terms

    Hard disk drives (also called hard drives or disk drives) is the mechanism that reads and writes data on a hard disk. Hard disk drives (HDDs) for PCs generally have seek times of about 12 milliseconds or less. Many disk drives improve their performance through a technique called caching .

    There are several interface standards for passing data between a hard disk and a computer. The most common are IDE and SCSI .

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    Hard drive

    Hard drive The role of the hard drive

    The hard drive is the component which is used to permanently store data, as opposed to RAM. which is erased whenever the computer is restarted, which is why the term mass storage device is sometimes used to refer to hard drives.

    The hard drive is connected to the motherboard using a hard drive controller which acts as an interface between the processor and the hard drive. The hard drive controller manages the drives linked to it, interprets commands sent by the processor and routes them to the drive in question. Hard drives are generally grouped by interface as follows:
    • IDE
    • SCSI
    • Serial ATA

    When the USB standard appeared, external cases which could connect a hard drive using a USB port were released, making hard drives easy to install and increasing storage capacity for macking backups. These are called external hard drives. as opposed to internal hard drives which are plugged directly into the motherboard ; still, they are the same disks, even though they are connected to the computer using a case plugged into a USB port.

    A hard drive is made up of not just one, but several rigid metal, glass, or ceramic disks, stacked very close to one another and called platters.

    The disks turn very quickly around an axle (currently several thousand revolutions per minute) in a counter-clockwise direction. A computer works in binart mode, meaning that the data is stored in the form of 0s and 1s (called bits ). Hard drives hold millions of these bits, stored very close to one another on a fine magntic layer a few microns thick, which is covered by a protective film.

    They are read and written using read heads located on both sides of the platters. These heads are electromagnets which raise and lower themselves in order to read or write data. The read heads are only a few microns from the surface, separated by a layer of air created by the rotation of the disks, which generates a wind of about 250km/h (150 mph)! What's more, these disks are laterally mobile, so that the heads can sweep across their entire surface.

    However, the heads are linked to one another and only one of them can read or write at a given moment. The term cylinder is used to refer to all the data stored vertically on each of the disks.

    This entire precision mechanism is contained within a fully airtight case, as the smallest particle can degrade the disk's surface. This is why hard drives are closed shut with seals, and the warning "Warranty void if removed ", as only hard drive manufacturers can open them (in particle-free "cleanrooms").

    How it works

    The read/write heads are said to be "inductive", meaning that they can generate a magnetic field. This is especially important in writing: The heads, by creating positive or negative fields, polarise the disk surface in a very tiny area, so that when they are read afterwards, the polarity reversal completes a circuit with the read head, which is then transformed by an analog-digital converter (ADC) into a 0 or 1 which can be understood by the computer.

    The heads start writing data from the edge of the disk (track 0), then move onward towards the centre. The data is organized in concentric circles called "tracks ", which are created by low-level formatting.

    The tracks are separated into areas (between two radii) called sectors. containing data (generally at least 512 octets per sector).

    The term cylinder refers to all data found on the same track of different platters (i.e. above and below one another), as this forms a "cylinder" of data.

    Finally, the term clusters (also called allocation units ) refers to minimum area that a file can take up on the hard drive. An operating system uses blocks . which are in fact groups of sectors (between 1 and 16 sectors). A small file may occupy multiple sectors (a cluster).

    On old hard drives, addressing was done physically, by defining the position of the date from the coordinates Cylinder/Head/Sector (CHS ).

    Block mode

    Block mode and 32-bit transfer are used to get the best performance out of your hard drive. Block mode involves transferring data in blocks, usually in 512-byte packets, which keeps the processor from having to process a large number of tiny one-bit packets. This way, the processor has the "time" to perform other operations.

    Unfortunately, this data transfer mode is only useful for older operating systems (such as MS-DOS ), as recent operating systems use their own hard drive manager, which makes this management system obsolete.

    There is a BIOS option (IDE HDD block mode or Multi Sector Transfer ) which can sometimes determine how many blocks can be managed at once. It is a number between 2 and 32. If you don't know it, there are several solutions available:
    • Check your hard drive's documentation
    • Search for the drive's specifications on the Internet
    • Carry out tests to determine it.
    Still, block mode may generate errors in certain systems, due to redundancies in the hard drive manager. The system involves disabling one of the two managers:
    • the 32-bit software manager in the operating system;
    • block mode in the BIOS.
    32-bit mode

    32-bit mode (as opposed to 16-bit mode) is characterised by 32-bit data transfers. 32-bit transfer is comparable to 32 doors opening and closing all at once. In 32-bit mode, two 16-bit words (groups of bits) are transmitted one after another, then assembled.

    The improvements in performance when switching from 16-bit mode to 32-bit mode are generally insignificant. In any event, it is no longer normally possible to select the mode, as the motherboard automatically determines which mode to use depending on the type of hard drive.

    However, automatically selecting 32-bit mode may slow down IDE CD-ROM drives whose speed is higher than 24x when they are alone on an IDE ribbon cable. Indeed, when a CD-ROM drive is alone on the cable, the BIOS cannot tell if it is compatible with 32-bit mode (because it is looking for a hard drive), in which case it switches to 16-bit mode. In this case, the transfer speed (incorrectly called the transfer rate ) will be lower than the one claimed by the manufacturer.

    The solution is to plug the CD-ROM drive and a 32-bit-compatible hard drive into the same ribbon cable.

    Technical specifications
    • Capacity. Amount of data which can be stored on a hard drive.
    • Transfer rate. Quantity of data which can be read or written from the disk per unit of time. It is expressed in bits per second.
    • Rotational speed. The speed at which the platters turn, expressed in rotations per minute (rpm for short). Hard drive speeds are on the order of 7200 to 15000 rpm. The faster a drive rotates, the higher its transfer rate. On the other hand, a hard drive which rotates quickly tends to be louder and heats up more easily.
    • Latency (also called rotational delay): The length of time that passes between the moment when the disk finds the track and the moment it finds the data.
    • Average access time. Average amount of time it takes the read head to find the right track and access the data. In other words, it represents the average length of time it takes the disk to provide data after having received the order to do so. It must be as short as possible.
    • Radial density. number of tracks per inch (tpi ).
    • Linear density. number of bits per inch (bpi ) on a given track.
    • Surface density. ratio between the linear density and radial density (expressed in bits per square inch).
    • Cache memory (or buffer memory): Amound of memory located on the hard drive. Cache memory is used to store the drive's most frequently-accessed data, in order to improve overall performance;
    • Interface. This refers to the connections used by the hard drive. The main hard drive interfaces are:
      • IDE/ATA
      • Serial ATA
      • SCSI
      • However, there are external cases used for connecting hard drives with USB or FireWire ports.

    For unlimited offline reading, you can download this article for free in PDF format:

    Hard Drive - Mac Guides

    The first hard drives were used in the IBM 305 mainframe. These drives had a capacity of 5MB-- a huge amount of storage for the time, when most files were measured in bytes. However, the drive was approximately the size of a washing machine. Impractical and expensive, hard drives would remain in datacenters out of reach of consumers.

    It was in 1980 that Seagate released the ST-506, the first 5.25-inch hard drive. It had the same capacity as the IBM hard drive introduced 25 years earlier, but was much smaller physically.

    Over the next two decades, hard drive technology rapidly advanced. Hard drives eventually came to be included with all personal computers, and capacities increased exponentially. Once capacity and speed were no longer major impediments, a standard form-factor emerged: 3.5" wide. Today, the largest hard drives exceed 1 TB (1024 GB), while commonly used hard drives range from 30 GB to 500 GB. Increasingly, hard drives are being replaced with Solid State Drives in laptops for size and power consumption reasons. While the hard drive is certain to be around for some time more, it is clear that SSDs are beginning to supplement hard drives as the preferred medium of storage.

    Hard drives are generally used as the primary place to store data. A computer's operating system is also stored on a hard drive, as well as installed applications and other files. Hard drives perform this role especially well because the number of times a sector may be read from or written to on a hard drive before it goes bad is much higher than for flash-based storage mediums.

    The original iPods. and current higher end iPods such as the the iPod classic use hard disk storage. Lower end iPods such as the iPod shuffle and the iPod nano use flash based storage.

    Sizes, Speeds and Formats

    Physical Size: Desktop machines commonly use 3.5 inch hard drives, while notebook machines usually use 2.5" drives. In addition to the width of the drive, there is also the height to consider. Most Apple notebooks can take a drive of 9.5 mm tall, but not drives that are 12 mm tall. This excludes some taller high capacity 2.5" models, which use additional platters for more data storage. Desktop 3.5" drives have been standardized on the "1/3-height" or 1 inch tall format for many years, it is unusual to encounter a hard drive in the 'full-height' or 'half-height' size.

    Speed: Hard drive speed is primarily listed as the rotational speed of the platters in RPM (there are other measures of performance as well, but real-world drive performance is difficult to judge from the numbers. For most purposes, rotational speed is a rough measure of the basic performance of a drive). The most common rotational speeds today are:

    • 4200 RPM -- notebook drives only
    • 5400 RPM -- standard for notebooks, below standard (or special power saving eco-drives) for desktops
    • 7200 RPM -- standard for desktops, premium for notebooks
    • 10,000 RPM -- premium for desktops, only offered by Western Digital in their SATA (Veloci)Raptor series and in SCSI and SAS server-class drives
    • 15,000 RPM -- found in high-end servers, only offered with SCSI and SAS (Serial attached SCSI) connections

    The market now provides drives with an amount of Flash (at the moment 4GB) embedded inside a hard drive, which caches the most used blocks on the Flash to provide a SSD-like performance when accessed, with the storage space and the cost of a 7200 RPM hard drive. The first manufacturer to offer this technology is Seagate in it's Momentus XT-series. As they are 9.5mm tall, they suite every Mac which is capable to fit 2.5" SATA-drives.

    Bus Format

    There are several different bus standards for connecting a computer to an internal hard drive.

    SCSI: The earliest connection used on Macintoshes, starting with the Mac Plus in 1986, was SCSI. This was faster than the drive connections offered in PC machines, and allowed a number of drives to be connected at once by daisy-chaining them. SCSI circuitry was expensive, however, and the termination and ID requirements of devices (which could be scanners, CD-ROMs, and printers as well as drives) could be troublesome. SCSI-1, which was used in Macintoshes, used a 50-pin ribbon connector internal interface. Macs with SCSI usually had a DB-25 external connector, and SCSI devices usually had Centronics-50 connectors.

    There are other variants of SCSI that have been developed over the years, and SCSI is still in use in high end server and workstation machines, where performance and drive self-management are valued higher than the cost. Premium SCSI drives can go up to 15,000 RPM; however, these are rarely used in the Macintosh environment.

    IDE/ATA: IDE drives (also known as ATA/33, ATA/66, ATA/133, ATA/133, UltraATA, PATA and Parallel ATA) use a simple circuit of Integrated Drive Electronics, and are controlled by the computer's CPU and drive controller chip - consequently they are less expensive than SCSI drives, but are slower. Desktop IDE drives use a 40 pin ribbon cable connector plus a 4 pin Molex connector for power. Notebook drives use a 44 pin connector that incorporates power. Depending on the version, IDE has a theoretical bandwidth of 33, 66, 100 or 133 MB/s. There is no external form of IDE.

    Serial ATA (SATA): To break the speed limitations of IDE standard, Serial ATA uses a serial connection with few pins, running at a very high speed, rather than IDE's parallel connection with many pins. Serial ATA interfaces have a theoretical bandwidth of 187.5 MB/s (SATA 1.5Gb/s), 375 MB/s (SATA 3Gb/s - sometimes called SATA2, which isn't strictly accurate) and 750MB/s (SATA 6Gb/s, so-called SATA3) Another benefit is that SATA cables are much smaller, and can extend longer distances. SATA busses are strictly one device per cable, and there is no daisy-chaining.

    Note: SATA 3Gb/s and SATA 6Gb/s drives are backwards compatible with SATA 1.5Gb/s busses.

    External Hard Drives

    External drives consist of a drive mechanism (either IDE or SATA) in an enclosure. The enclosure provides power to the drive, and has a bridge circuit that translates between the drive's interface (IDE or SATA) and one or more of USB. Firewire or eSATA. It is important to understand that the hard drive mechanisms inside external hard drives are identical to the internal drives used in Macs.

    There are many companies who sell ready-made external hard drives: LaCie, Seagate, Western Digital, Maxtor, OWC, Iomega, Iogear, Wiebetech and others. There are also empty enclosures from a variety of companies including MacAlly, Firmtek, Coolmax, OWC, and others that you can install your own drive mechanism into. Any enclosure will be designed to support one of IDE or SATA internal drives; very few can accommodate both types.

    USB: the simplest, least expensive and slowest of the interfaces. USB 1.1 is almost never used for hard drives because of its slow speed. USB 2.0 is faster (theoretical speed 480 Mb/s) however in real life USB 2.0 cannot sustain its full rated speed. USB 1.1, 2.0 and 3.0 connectors are compatible with one another. USB is most commonly used with portable external drives, flash-memory based drives (where speed is limited by the flash memory) and drives that are used for PC machines. Apple machines have had USB interfaces since the introduction of the iMac G3, however USB 2.0 didn't become standard across the line until late 2003. PowerPC Macs cannot boot from an external USB 2.0 hard drive while Intel Macs can; however, Apple only officially supports booting off of a FireWire drive.

    Firewire 400 (IEEE 1394a, iLink) and FW 800 (IEEE 1394b): Developed by Apple, Sony and others, Firewire is a fast and reliable interface well suited to hard drive use. Multiple drives can be daisy-chained on Firewire bus. Firewire 400 has a nominal speed of 400 Mb/s and FW 800 double that. In practice, Firewire 400 is anywhere from 20% to 80% faster at sustained data transfer than USB 2.0, while Firewire 800 is faster than 400, but not truely double the real world speed. Firewire is the preferred interface for Macintosh external drives. FireWire 400 was introduced on the Power Mac G3 Blue and White in 1999 and has been standard on all Macs built since the year 2000. Any Mac with a Firewire port can boot from an external Firewire hard drive.

    eSATA (external Serial ATA): eSATA is an external variant of the SATA drive interface. It offers the same nominal speed as internal SATA drives (1200 Mb/s or 150 MB/s for SATA-150). eSATA uses an "I-Type" connector which is more durable than the internal "L-Type" and can have longer cable runs. The eSATA connector is more robust and the cable can be longer, but the signal is the same. L-type and I-type connectors can be adapted to each other using the appropriate cables. eSATA cannot be daisy-chained; it is one cable per drive. There are some new eSATA enclosures and cards which support Port Multiplication. which allows multiple drives on a single eSATA cable.

    No Maintosh has incorporated eSATA connections, but third party eSATA cards are available for PCI bus (PowerMac G4. PowerMac G5 pre-Oct. 2005), PCMCIA bus (Powerbook G4 15" and 17"), PCI-e bus (PowerMac G5 Oct. 2005 and later, MacPro ) and ExpressCard/34 (MacBook Pro ).

    At the time of writing, only certain G5 machines are bootable from eSATA drives, and only if the interface card supports booting.

    Portable enclosures: These are external enclosures for 2.5" hard drives, either IDE or SATA, with USB or Firewire connections. Most USB external 2.5" enclosures do not have AC power supplies, but rely on the 5V/500 mA power available on the USB port. Unfortunately, some of these enclosures draw more than 500 mA, and Apple strictly limits the power available from the USB ports, which sometimes causes USB powered drives to not work properly on Macs. When you have a choice, pick a drive or enclosure with its own AC power.

    There are also 1.8" "micro" hard drives in USB enclosures available in sizes from 5 to 40 GB. These are generally safe to power from USB ports. Their popularity is declining with the rapid drop in price of large flash-memory USB keychain 'drives' which have reached 32 Gb in capacity.

    Bridge Chipset: An external drive relies on a 'bridge' circuit to manage the translation between the external bus (USB/FW/eSATA) and the drive interface bus (IDE/SATA). Compatibility and performance of the external drive depends largely on the quality of the chipset used in the bridge. The Oxford chipsets have the best reputation.

    Keep in mind that older enclosures may not be compatible with larger drives, and their power supplies may not provide enough stable power for drives larger than 200 GB. Check the compatibility of the enclosure and the chipset before considering loading a new drive in an older enclosure.

    NAS or Ethernet - Drives connected by an Ethernet connection are often called Network Attached Storage or NAS. These act more like a file server than a hard drive. Disk Utility is NOT used to format a NAS device; usually the device has its own built in operating system and formatting method. Macs address the space on a NAS device as a network server, usually using SMB, but occasionally Apple's AFP is supported.

    Comparison with flash based storage

    In the past platter-based hard drives have stored considerably more data than comparably priced flash storage devices. However, in recent years the gap has closed somewhat. Apart from capacity differences, SSD flash-based storage is considered by most to be superior because it is usually faster and is more resistant to failure resulting from sudden movements (especially relevant for portable devices). However, SSD drives are more expensive than comparable platter-based drives. If SSD drives were to approach the capacity to cost ratio of platter-based hard drives, SSD would become a much larger player in the hard drive market.

    Not all types of Flash memory are suitable for hard drive replacement. The less-expensive forms of Flash memory (such as the common types found in USB keychain drives and camera memory cards) have architectural issues with random writing and erasing speed, which makes them more suitable for occasionally-accessed storage, rather than continually-accessed 'hard' drive use.

    Hard Drive Choices for Macintoshes

    Early Macintoshes used SCSI hard drives. Starting in 1995 Apple started phasing in IDE (Parallel ATA) hard drives, which had been used for several years on PC machines. These drives lacked the on-board drive management 'intelligence' of SCSI drives, and could have a maximum of 2 drives (Master and Slave) on a IDE bus. But IDE drives were considerably less expensive than SCSI, so they were introduced first in the consumer Performa models. With the introduction of the Blue and White G3 towers, IDE was used throughout Apple, and SCSI became only available with an optional PCI add-in adaptor card. Starting with the G5 machines, Apple switched from IDE to Serial ATA hard drives, although the Powerbook and iBook lines remained on 2.5" IDE drives until the end. Currently, IDE is still used for CD- and DVD-RW drives.

    Drives for Mac models: Installing or upgrading hard drives in a Mac is limited by the bus type, size, and number of bays available for installation.

    • Intel Mini - one 2.5" SATA **
    • Intel iMac - one 3.5" SATA
    • Intel MacPro - four 3.5" SATA
    • MacBook and MacBook Pro - one 2.5" SATA **
    • MacBook Air - one 1.8" IDE drive OR one Solid State flash drive
    • PowerMac G5 - two 3.5" SATA (Some companies produce brackets for mounting 2 to 4 additional drives internally. These would require additional SATA or IDE adaptor cards.)
    • iMac G5 - one 3.5" SATA
    • iMac G3/G4 - one 3.5" IDE
    • PowerMac G4 - various numbers of 3.5" IDE depending on model *
    • eMac G4 - one 3.5" IDE
    • Mini G4 - one 2.5" IDE **
    • PowerBook G3/G4 and iBook G3/G4 - one 2.5" IDE **
    • PowerMac G3 Beige - one or two 3.5" IDE*. All models have both IDE and SCSI busses, early models were restricted to one IDE drive per bus rather than 2

    (*) Desktop Macs earlier than the 2002 Quicksilver PowerMac G4 were limited to IDE drives of 128 GiB in size or less. This is a limitation of the IDE controller on the Mac logic board. You cannot get around it by partitioning the drive. To use larger hard drives, there is third party software that can patch the problem, or you can install an IDE or SATA drive controller in a PCI slot and connect the drive to this card, or you can install a larger drive in a Firewire enclosure to use it with these machines. The chipset in the enclosure must be able to support the drive, which is not generally an issue except with very early (and thus, slow and unstable) chipsets. Assuming the chipset supports the drive, the FireWire bus itself will support any size drive.

    (**) Notebook and Mini drives must be 9.5mm high or less. All unibody Apple laptops (from late 2008 and onwards) and the earliest model PowerBook and iBooks plus pre-unibody 17" MBP can fit a 12.5mm hard drive.

    Installation

    Installation of hard drives is relatively simple in Apple tower machines. In G5 and MacPro towers, the cables, connectors and mounting screws are provided already in position for a new drive. The iMac series are moderately difficult.

    The difficulty of installation on Apple notebooks varies widely, from easy (MacBook) to time consuming and difficult (iBook G3). iFixIt.com have some good takeapart tutorials.

    Installing an external hard drive is as simple as plugging it in.

    A new hard drive has to be formatted (initialized, or Erased) with Disk Utility before it will appear on the desktop. The usual choice of formatting is "Macintosh Extended (HFS+)", although if a drive will be used interchangeably for Bootcamp and Mac, it might be formatted "MS-DOS (FAT32)". A drive can be Partitioned at this time, allowing for two or more separate Volumes, which can be formatted, named and mounted separately. Formatting or Partitioning a drive will destroy any data on that drive.

    External drives may come already formatted to Windows' proprietary NTFS format - Macs don't write properly to NTFS, so the drive should be reformatted with Disk Utility before using it.

    When you install a hard drive, you'll see that the number of GB reported by OSX is about 7% lower than the advertised drive capacity in GB -- this is normal, read the article Hard Drive Size Discrepancy for more detail. Note that in Snow Leopard this is not the case. You'll always see the advertised disk capacity in Finder because Snow Leopard uses a different way of calculating this.

    Once you have a second hard drive installed and formatted, it will appear as an icon on your Desktop. If you attach an external drive, it will show up on the Desktop as well. Copying files to the second drive is just dragging and dropping the file or folder to the icon of the new drive. When you are doing an Open or a Save from a program, the second drive will show up as an option of where to open or save from. Unlike Windows, Mac drives are not assigned drive letters (C. D: etc) but they are known by the name you give them, which you can change on the Desktop.

    When turning off or unplugging an external drive, first drag the Desktop icon of the drive to the Trash to unmount it. Suddenly disconnecting or turning off a mounted drive can cause data damage. When you are working with Firewire 400 drives, do not plug or unplug the cables when the machine is running - always shut down the machine and the drive before disconnecting or connecting Firewire, to minimize the chance of damage.

    Formatting

    Hard drives (and other storage devices like USB keychain drives) must be formatted before data can be read and written. There are several different formatting methods that can be used, and each has its advantages and drawbacks.

    Macintoshes since the era of System 8.1 use the HFS+ (Hierarchical File System Plus) format. In order to be a bootable drive for Mac OS X, a drive should be formatted HFS+ (Journaled).

    (Note: an Intel Mac cannot boot from a hard drive that was initially formatted on an earlier PowerPC Mac without a GUID partition table .)

    HFS+ cannot be read by Windows machines (or by a Boot Camp Windows installation on a Mac). There is commercial software, MacDrive which allows Windows to read and write HFS+.

    Windows machines use both FAT32 and NTFS formats.

    FAT32 (File Allocation Table 32 bit) is readable and writable by both Mac and Windows operating systems. It is a good choice for storage, such as USB keychain drives, that have to move between systems.

    However, Windows artifically limits the size it can format FAT32 volumes to 32 GB (but you can format a FAT32 volume using Apple's Disk Utility - choosing the "MS-DOS" option - to any size). FAT32 also has a 4 GB limitation of the maximum size for an individual file. Normally this wouldn't be a concern, but it can get in the way of video projects and backup software, which create large single files.

    NTFS is a proprietary Microsoft format, and is not an open standard. Macintoshes cannot write to NTFS formatted volumes, and reading from NTFS with a Mac is not 100% reliable. There is some software for Mac OS X that allows reading and writing from NTFS; MacFuse NTFS-3G are open source, Paragon NTFS is commercial

    Once in a while you run into FAT16, mostly in very old drives or older flash memory cards. FAT16 is limited to 4 GB total volume size, and it not used for computer hard drives any more. If you have an older digital camera, do not format the memory card in a computer, always use the camera itself to format the cards.

    Partitioning

    When a hard drive is set up in Disk Utility, you have the choice of dividing one hard drive into two or more logical partitions. These partitions will show up as separate icons on the Desktop, with separate volume names. Each partition can be formatted to a different formatting (HFS+, FAT32).

    Typically, partitioning a hard drive is destructive, i.e. the existing data on the hard drive will be lost. Boot Camp Assistant is a specialized, non-destructive partitioning tool provided with OS X 10.5 and 10.6 that will add a partition so that you can install Windows on your Mac. Disk Utility in OS X 10.5 and 10.6 can also resize certain partitions non-destructively. In any case, it is highly advisable to have a verified backup before using any tool to repartition a hard drive, even with a non-destructive tool.

    Backing Up

    Fact: Every hard drive in the world will fail, sooner or later. And many times they will quit without giving any prior notice.

    Having a backup of your Mac's internal drive on a second drive (internal or external) is an excellent idea. It's even better if the backup has a working OS on it. If the main drive has a problem, then you can boot from the external drive to continue using the machine, and to do diagnostics and repairs on the main drive.

    A backup can be made as simply as dragging files from one drive to another. However, remembering to do it is usually the downfall of manual backup plans. Using some backup software (below) can add scheduling and flexibility.

    Backup Software

    • Carbon Copy Cloner free (by donation) backup software that can create bootable clones.
    • Retrospect commercial software that can do scheduled and scripted backups to a wide variety of media, and back up both Macs and Windows machines over the network.
    • SuperDuper free trial software that can make clones of a hard drive, the shareware paid version adds scheduling and scripting of backups.
    • TimeMachine is included with OSX 10.5 and offers hour by hour backup of a Mac's hard drive to an external drive.

    Installing an operating system OSX can be installed on more than one drive on a single machine. All that's necessary is to either make a bootable clone of an existing drive (with CarbonCopyCloner or SuperDuper), or to run the installer from the OSX DVD, and target the second drive.

    Checking a hard drive's health

    The Disk Utility program has a readout called S.M.A.R.T. status, which is a drive self-test status report. Click on the icon of your hard drive on the left, and at the bottom of the window will be the status. If your S.M.A.R.T. status says anything but Verified, make a backup of your data right away, because a failing S.M.A.R.T. status indicates a drive that is breaking down, and it may fail at any time.

    Disk Utility also allows you to do a "Verify Disk" operation on a hard drive to check for problems, and a "Repair Disk" to fix some kinds of catalog and data corruption on a hard drive. Repair Disk can't fix a hardware failure on a drive, but thankfully most drive problems are issues with the data, and can be repaired in software. If Disk Utility can repair it, sometimes a commercial program like DiskWarrior or TechTool Pro can.

    You can't Repair the drive you are booted from, so this is where it helps to have a bootable OS on a backup drive. Alternatively, reboot with your OSX DVD inserted, holding down the "C" key to force the Mac to boot from the CD or DVD. Then, instead of going into the OSX installer, choose Disk Utility from the Utilities menu at the top, and run the Repair Disk on your hard drive from there.