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 (ie stores data even when unpowered) data storage device used in computers. Data is stored on a magnetic surface employing multiple spinning platters and moving read-write heads.

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History

Hard drives were first used in computers in 1955, with the introduction of the IBM 305 computer. The hard drive used had a capacity of 5 MB.

However, it was not for several more decades that hard drives became practical for most uses, due to their large physical size. In 1980, Seagate introduced 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.

Over the next two decades, hard drive technology rapidly advanced. Hard drives eventually came to be included with all personal computers, and their capacities increased exponentially. For example, the first 100 MB hard drive was introduced in 1991, but just 4 years later, in 1995, came a 2 GB hard drive. The largest hard drives today exceed 1 TB (1024 GBs), while commonly used hard drives range from 30 GB to 500 GB.

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Uses

Hard drives are generally used as the primary place to store user data. A computer's operating system is also stored on a hard drive, as well as installed applications and other files.

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.

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Sizes, Speeds and Formats

Physical Size: Desktop machines commonly use 3.5 inch hard drives, 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 are:

  • 4200 RPM (notebook drives only)
  • 5400 RPM (standard for notebooks, below standard for desktops)
  • 7200 RPM (standard for desktops, premium for notebooks)
  • 10,000 RPM (premium for desktops, only offered by Western Digital in their Raptor series and in SCSI server-class drives)

Bus Format: There are several different bus standards for connection to hard drives.

SCSI: The earliest commonly used on Macintoshes, starting with the Mac Plus in 1986, was SCSI (Small Computer Serial Interface). 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 were 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 are seldom 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. Desktop IDE drives use a 40 pin ribbon cable connector plus a 4 pin Molex connector for power. Notebook IDE drives use a 44 pin connector that incorporates power. There are no external IDE connections, IDE is intended for internal use only. Depending on the bus, IDE has a theoretical bandwidth of 33, 66, 100 or 133 MB/s

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 interaces have a theoretical bandwidth of 150 MB/s (SATA-150) and 300 MB/s (SATA-300 - sometimes called SATA2, which isn't strictly accurate). Another benefit is that SATA cables are much smaller, and can extend longer distances. SATA busses are strictly one device per cable, there is no daisy-chaining.

Note: The distinction between SATA-150 and SATA-300 is pretty much academic, as no drive mechanism made to date is fast enough to saturate a SATA-150 bus, much less SATA-300 (SATA-300 may be of some benefit if there is a hardware based multi-drive array at the other end). SATA-300 drives are backwards compatible with SATA-150 busses.

Serial ATA and IDE drives are not compatible with each other's bus.

There are two standards for external SATA connections, the original "L-type" connector, and eSATA, which is now the standard, with a locking "I-type" connector. 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. No Maintosh to date incorporates eSATA connections, although third party interface cards are available for PowerMac G3, G4, G5 and MacPros (variously PCI, PCI-X and PCI-e cards), 15" and 17" PowerBook G4 (PCMCIA) and MacBook Pros (Expresscard/34). No other Macs can be adapted to eSATA.

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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's 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 (USB 2.0): 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 (nominal speed 480 Mb/s) however in real life USB 2.0 cannot sustain its full rated speed. USB is most commonly used with portable external driver, 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, however Intel Macs can.

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 double the real world speed. Firewire is the preferred interface for Macintosh external drives. FW 400 was introduced on the Power Mac G3 Blue and White in 1999 and has been standard on all Macs since later 2000. Any Mac with a Firewire port can boot from an external FW 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.

At this time only certain G5 machines are bootable from eSATA drives, and only if the interface card supports booting. eSATA cannot be daisychained; it is one cable per drive. There are some new eSATA enclosures and cards just being released (late 2007) with Port Multiplication, which promises to support multiple drives on a single eSATA cable.

Portable enclosures: There are external enclosures for 2.5" hard drives, either IDE or SATA, with USB or Firewire connections. Most USB extenal 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, so sometimes USB powered drives do 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" drives in USB enclosures available in sizes from 5 to 40 Mb. 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.

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Comparison with flash based storage

In the past 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 sudden movements (especially relevant for portable devices). But it is still ferociously expensive. If SSD flash based drives were to approach the capacity to cost ratio of hard drives, they would be a formidable threat to hard drives.

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.

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Hard Drive Choices for Macintoshes

Early Macintoshes used SCSI hard drives. Starting in 1995 Apple started phasing in IDE 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 Gb 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, or you can install a larger drive in a Firewire enclosure to use it with these machines. The Firewire bus is not subject to the same limitation as the IDE drive controller on the motherboard.

(**) Notebook and Mini drives must be 9.5mm high or less. 12 mm high drives will not fit, other than in the earliest model PowerBook and iBooks.

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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.

Any 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.

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.

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See Also

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External Links

Drive Manufacturers

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