Hard Drive

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A hard drive (also known as a hard disk, hard disk drive, HD or HDD) is a non-volatile data storage device used in computers. Hard Drives are a non-volatile form of data storage, meaning that even when there is no power to the drive data is retained. Data is stored on a magnetic surface employing multiple spinning platters and moving read-write heads.



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 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.


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.


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.

See Also

External Links

Drive Manufacturers