Hard Disks

All modern computers incorporate a hard disk drive (HDD) or hard disk unit (HDU), which contains documents, applications, much of the operating system (OS) and temporary files too large for the random access memory (RAM).

The capacity or size of a drive is given in megabytes (MB), gigabytes (GB) or terabytes (TB). You can convert the given figures into bytes by using the following rules:-

Unfortunately, some third-party manufactures define 1 MB as a million bytes and 1 GB as a billion bytes. Hence a non-Apple drive that appears similar to an Apple product may actually have a lesser capacity.

The given capacity usually includes the space needed for formatting and for any invisible files that are used by the system. Most operating systems also impose a restriction on the maximum size of any volume, the latter defined as an entire drive or a partition of a drive. Modern versions of the Mac OS set this at over 2 TB.

A typical modern internal drive accommodates at least 20 GB of data. Older drives of 40, 80, 160, 240 or 500 MB and 1, 2, 4 or 6 GB aren’t usually worthwhile, especially since recent operating systems often use over 200 MB for their files, to which you must add the space required for your own applications and documents.

Basic Principles

A drive is either fixed, meaning that you can’t remove the medium containing the data, or uses removable media, usually a cartridge or disk. Fixed drives use Winchester technology, with several rigid and magnetically-coated disks known as platters stacked one above the other and mechanically linked together, effectively creating a single recording surface.

Data is transferred to and from the disks using heads placed above and below each platter, effectively doubling the surface area. Older drives might use eight platters and sixteen heads, whilst modern devices often have three platters and six heads.

The following terms are often encountered when dealing with drives:-

All the platters in a typical drive spin together at 3,600, 4,200, 4,800, 5,400, 7,200, 10,000 or 12,000 revolutions per minute (rev/min or rpm). Each disk contains between 400 and 2,000 tracks per inch (tpi) and is formatted to have a specific number of sectors per track (SPT), often 36. A typical drive employs a data density of between 7,000 and 60,000 bits per inch (bpi) and can convey data at a transfer rate of 5 to 40 MB per second (MB/s), or even higher.


The speed of a drive is determined by its head mechanism, the rotational speed and size of its platters, its capacity and the quality of its driver software. This means that a low-cost drive of small diameter with good software can out-perform a larger drive. Note that the speed may also be limited by the electrical interface that connects the drive to the computer.

Generally speaking, a drive with many small platters is faster than a large drive with fewer platters. However, when working with multimedia applications that need a continuous flow of high-speed data, usually involving the recording of sound and pictures in real-time, you must have a drive with a high rotational speed. With desktop machines a speed 7,200, 10,000 or 12,000 rev/min is preferred, whilst 5,400 rev/min should suffice for the small drive in a portable computer.

The average seek time, measured in milliseconds (ms), indicates the time taken to reach a given sector. This is a guide to a drive’s general performance and should be 15 ms or less. A fast modern drive usually has a seek time of under 8.5 ms.

PC Complications

If you’re unfortunate enough to own an old PC you may find that the BIOS (Basic Input-Output Operating System) limits the maximum number of cylinders to 1024, as well as allowing only 16 heads, 63 sectors per track and 512 bytes per sector. This restricts the maximum size of any drive to:-

1024 × 16 × 63 × 512 ​= 528,482,304 bytes ​= 504 MB

A technique known as translation, usually in the form of Logical Block Addressing (LBA), can remove this cylinder limit, fooling the BIOS into thinking there are fewer cylinders and that a higher number is used for one of the other parameters. Alternatively, an EIDE controller card avoids the problem by using a form of BIOS that’s built into the card itself.


The mechanics of a Winchester drive are simple and therefore robust. However, its reliability depends on the electronic hard disk controller that’s normally built into the drive’s logic card.The average life of a modern drive, given by its mean time between failures (MTBF) is typically 50,000 hours, or around 5.7 years of continuous use. This assumes the drive is switched on and off only twice a day to minimise any thermal shock that would otherwise shorten its life.

Never dismantle a hard disk drive unless you’re prepared to take the risk of damaging it.

Disk Diameter

The small disks in a miniature drive must rotate a higher speed to obtain the same data transfer rate as a large drive. However, the heads in a small drive travel shorter distances, giving faster access than a large drive. Better still, the low mass of small disks makes them less prone to mechanical shock, although they do put greater demands on disk technology.

Disks come in various sizes, including diameters of 1.8, 2½, 3½ or 5¼ inch. A very small drive with a capacity of 40 to 220 MB may have 3 to 5 platters, each 3½ inches in diameter. However, modern laptop computers are normally fitted with a tiny 2.5 inch, 1.8 inch, 17 mm or 19 mm drive, whilst Apple’s iPod MP3 player also has a 1.8 inch drive. And Toshiba has recently announced the introduction of even smaller drives with a diameter of only 0.85 of an inch.

A large drive often incorporates 5¼ inch diameter disks, although 8 or even 14 inch drives can be encountered. Unfortunately, earlier 5¼ inch drives, originating from older machines, are painfully slow compared to modern devices. Older 1 GB drives are sometimes better, containing up to 11 platters, often in a full-height package (see below).

Parking and Head Crashes

Whilst a hard disk is spinning each head floats above its platter on a cushion of air, ensuring that the heads or platters don’t get worn. When not in use, the heads withdraw from the platters and move into a landing zone. This process, known as parking, protects the heads and disks from damage that might be caused by any mechanical shocks during transportation.

A modern voice coil drive is usually self-parking, meaning that it automatically withdraws its heads when de-powered. An older drive that employs a stepper motor isn’t self-parking, instead relying on parking instructions supplied by the computer itself. Most modern computers produce such signals on shutdown, but not if you simply turn off the machine.

A serious fault in the mechanism or software can cause the platters to stop while the heads are engaged, causing the protective air cushions to disappear and the heads to come into direct contact with the platters. Such a head crash can damage the surface of the platters, although low-level formatting of the drive can often ‘map out’ any damaged sectors.

It’s also possible for a head to stick to a platter, causing erratic behaviour. A gentle thump to the drive may provide a temporary remedy, but you must then backup your data and contact an expert.

Other Aspects

If a drive has been used ‘upside down’ it may fail when correctly oriented, so you should always check that a drive is correctly mounted. With an internal drive the fixing arrangements usually ensure the positioning is in order. If in doubt, install the drive with the printed circuit logic card at the bottom. Fortunately, using a drive on its side isn’t a problem.

Internal Drive Types

Most modern computers have a Serial ATA or ATA/EIDE hard disk drive, although older Mac OS machines contain a SCSI drive whilst others have an IDE drive. Some Macintosh G4 models also have an internal FireWire port for a suitable drive.

The following types of internal drive may be encountered:-

Serial ATA (SATA)

This type of drive is used in the Apple G5 and later machines, giving a transfer rate of 150 MB/s, although future devices should reach 300 or 600 MB/s. The data, as 250 mV differential signals, travels along four wires and a 7-pin plug, which can be ‘hot-plugged’. Each drive is connected to its controller via a separate cable, which can be up to one metre in length.

All drives, irrespective of size, are connected using identical cables. Jumpers or links, as fitted on ATA/EIDE drives, aren’t required, as only one drive is wired to each cable. And, unlike older drives, ribbon cables aren’t employed, so the associated ventilation problems are also avoided. Three supply voltages are used: 12 V, 5 V and 3.3 V.


An Enhanced IDE (EIDE) drive, as found in many Macs and PCs, is a development of the original Integrated Device Electronics (IDE) drive, so called because it contains an integral drive controller. This controller is similar to that found in a SCSI drive (see below), but doesn’t apply the error checking that’s employed in SCSI, making it slightly less robust. EIDE drives are invariably cheaper than SCSI drives, although the performance of older models may be slightly inferior.

Modern drives usually conform to the Advanced Technology Attachment (ATA) standards, known collectively as ATA/EIDE, and sometimes as Parallel ATA (PATA) to avoid confusion with modern SATA drives. The interface itself is a subset of the system employed for Industry Standard Architecture (ISA) slots, as found in many PCs.

IDE drives are normally wired via a 40-way connector and matching ribbon cable, although some miniature drives employ a smaller 50-pin connector. Full-sized drives that conform to the ATA-5 standard or higher (see below) also have a 40-way connector, although they require an 80-conductor cable, which has ground wires interposed between each of the signal wires.

The number of devices that can be connected onto each circuit is dependent on the controller in the IDE controller card, or more usually the controller built into the machine. Older controllers only allow two IDE drives to be connected the bus circuit, with each device configured either as a master (for startup) or slave. Unfortunately, the slow access of a CD-ROM drive can reduce the speed of other drives using the controller. More modern controllers that conform to ATA standards (see below), permit up to four drives to be daisy-chained, in much the same way as SCSI devices.

Several varieties of ATA/EIDE device exist, as shown in the following table:-

4 - 8
Fast IDE
(LBA *)
4 - 16.6
Fast ATA
Fast IDE
Ultra ATA/33
Ultra DMA
Ultra ATA/66
(80 way cable)
Ultra ATA/100
(3.3 V Logic)
-Ultra ATA/133
-Ultra ATA/166

* Logical Block Addressing: accommodates capacities above 504 MB and up to 8.4 GB.

Self-Monitoring, Analysis and Reporting Technology: continually checks the drive and informs user of any problems.

ATA Packet Interface: incorporates adaption of SCSI command set for special devices such as CD-ROM drives, also allowing several drives to share a single bus.

You should always check that your hardware and software can support a particular type of drive. In addition, you may need to fit a special EIDE controller card in a spare PCI slot.


Small Computer System Interface (SCSI) drives are found in older 680x0-based Macs and some other computers. In common with a ATA/EIDE drive, SCSI also employs a disk controller. Connections are made via a 50-way connector and ribbon cable.

Depending on the variation of SCSI that’s used, your drive should transfer data at 20, 40 or even 80 MB per second (MB/s). You should check your computer hardware and software before using a drive that uses an improved version of SCSI, such as SCSI-2 (Fast SCSI) or SCSI-3 (Ultra SCSI), and variations such as Fast-Wide SCSI or Ultra-Wide SCSI.

For really fast data transfers you could consider using a drive that uses Ultra 2 SCSI, also known as low-voltage differential (LVD) SCSI, or one of the even faster variants of SCSI, such as Ultra 80 and Ultra 160, offering transfer rates of 80 and 160 MB/s respectively. The disks in these drives usually rotate faster than those in older devices, typically at 10,000 rev/min.

Although most kinds of SCSI are compatible you won’t get the best results if your SCSI drive is used with the wrong type of computer. For example, a LVD drive usually works with an older computer, but its speed is limited by the machine’s SCSI bus. Fortunately, connections for an improved form of SCSI can be provided by fitting a special SCSI card in any spare PCI slot in your computer. In fact, some Apple machines were supplied with a modern SCSI card and matching internal drives.

ID Numbers

The original version of SCSI only allows seven devices to be connected to each circuit, apart from the computer itself. Each device has a unique SCSI ID number between 0 to 7, although improved versions of SCSI permit 16 devices with numbers from 0 to 15. Whatever form of SCSI is used, one number, usually 7 or 15, is automatically assigned to the controlling computer.

Some machines have a separate internal bus and external bus, making all ID numbers available to internal drives as well as to external devices connected via a SCSI port. Others have a shared SCSI bus for both sets of connections: since all machines have a hard disk, usually accompanied by a CD-ROM, the available ID numbers can fall to six or less.


The FireWire interface is extremely fast and very easy to connect, although the speed can be limited by older drive technology. For example, some earlier FireWire drives contain an Ultra ATA drive and a bridge chip for FireWire. The latter often runs at only 12 Mbit/s, although the more recent Oxford Technology 911 bridge operates at 38 Mbit/s. This means that the FireWire interface can be faster than the drive itself, although later devices have overcome this limitation.

Expansion Bay

Portable machines often have an expansion bay, allowing you to install a hard disc, CD, DVD or removable cartridge drive. Although the connector and bay dimensions aren’t standardised, most employ ATAPI technology (see above), requiring matching software. Note that applications or other software designed for a SCSI drive isn’t suitable for this kind of device.


A small drive, with a capacity of 1 GB or more, can be incorporated within a PC Type III Card. Before getting one of these, check that the drive is fully compatible with the slots in your computer. You’ll also have to install special software.

Obsolete Drives

Some types of drive simply aren’t suitable for connection to a modern computer. Such a device may employ a serial interface, such as the original Apple HD20 external drive, or may require an external controller card. Many early PCs incorporate a Shugart ST506 interface, which employs a common 34-way ribbon cable for the control signals to all drives and individual 20-way cables for the data to each drive: up to four devices can be connected in a ‘daisy-chain’.

The oldest ST506 drives use modified frequency modulation (MFM), sometimes known as double-density recording, usually with 17 sectors per track and 512 bytes per sector. The data transfer rate is around 5 MB/s and each drive holds 100 MB or more.

More modern varieties, usually under 60 MB in size, employ run length limited (RLL) recording, an improved form of MFM that increases the data density by 50%, with 26 sectors per track and a transfer rate of around 7.5 MB/s. Finally, there’s Enhanced Small Device Interface (ESDI), a double-density form of MFM, with 34 sectors per track, giving 10 to 15 MB/s.

Fitting an Internal Drive

Before fitting a new drive, you should check if your machine can accept a Serial ATA, ATA/EIDE or SCSI device. Apart from the data connections, the installation procedure for all types of drives is similar.

Always consult an expert if you have any doubts about the installation of an internal hard disk drive.

Drive Connections

A drive’s connections are dependent on the type of drive and its interface (see above). The following applies specifically to conventional EIDE and SCSI drives, although the general procedures apply to any drive. Having said this, you can’t connect a drive with one kind of interface to a machine designed for a different connection unless you have a suitable adaptor.

The logic card on an EIDE or SCSI drive incorporates a data plug, designed to match the ribbon cable used for the drive’s interface (50-way for a SCSI drive or 40-way for an EIDE drive), as well as a power plug. A typical EIDE drive is illustrated below, showing two varieties of power connector, although only one type is normally fitted.

If you’re lucky, the computer will have a data cable fitted for an extra drive. If not, you’ll have to install a ribbon cable between the drive and the computer’s motherboard. If there isn’t a spare socket on the motherboard you’ll need a special splitter cable (in place of the standard ribbon cable) to tap into the connection of an existing drive. Similarly, you may need an extra power cable (or a special power supply splitter cable) to connect the drive to the motherboard’s power supply.


Most drives have links or jumpers that can be moved to select various settings or options. In a SCSI drive these usually include settings for the drive’s SCSI ID number and SCSI termination. If your drive doesn’t have any links it’s probably designed as a startup drive with an ID number of 0. It’s impossible to change the ID of this kind of drive.

The links are usually on the logic card or at the front of the drive, often to the left of the ‘activity lights’ connector. Unless instructed otherwise, leave them alone. And before making any changes, carefully check the manufacturer’s information. You should then install, remove or relocate the jumpers as instructed. To ensure that jumpers are always available you should ‘park’ any spare jumpers onto unused pins, making sure that only one side of each jumper is pushed onto a pin.

The jumpers on an ATA/EIDE drive can be quite confusing, as shown in this example:-

This shows three combinations of links. Each link has the following function:-

In practice, the master drive often only has the first link whilst a slave doesn’t have any links at all.

External Drives

An external drive is really an internal drive and power supply fitted into an enclosure and provided with a suitable interface. The latter is usually a Universal Serial Bus (USB) or FireWire connection, although older Mac OS computers require a SCSI connection. Before buying any external drive you should check that its suitability for you machine.

When fitting an unboxed drive into an enclosure you must consider the height of the drive. The usual heights include:-


The following types of external drive may be encountered:-


This kind of connection is limited in speed, although it’s convenient for connecting a basic backup device to a non-demanding machine. All you need do is plug the drive into your computer, ensuring that any external power supply, if used, is also connected. You don’t even have to shut down the computer before connecting the drive. Then you must then install the necessary driver files on your computer: in the Classic Mac OS these go in the Extensions folder, inside the System Folder.


This is a very fast interface suitable for the most demanding disk drive. As with USB, you just plug the drive in, install any required driver software and use it. Although FireWire can handle rates as high as 400 Mbit/s, some drives are really Ultra ATA devices of limited speed capacity fitted with a bridge chip (see above).


This interface, found on ‘classic’ Macs, is very tricky to use. The settings required for a drive’s SCSI termination and its SCSI ID number are determined by switches on the device’s rear panel or sometimes by internal links on the drive’s logic card.

The connections to an external SCSI drive are provided via one or two 50-way Amphenol (C50), 50-way micro D (HC50) or 68-way micro D (HC68) connectors. Some older devices have 25-way D (DB25) connectors, to match those fitted on some Apple computers. Some low-cost drives even have a convenient flying lead that plugs directly into the back of such a machine.

Other Devices

Removable Drives

A removable drive shouldn’t be confused with removable media. In this kind of device the entire drive, complete with heads, is removed for safe-keeping. Sizes range from a 3½ inch 425 MB drive to 1.6 GB or more. Older types of removable drive have been largely superseded by optical disks.

Redundant Array of Independent Disks (RAID)

RAID uses several drives to create a large, high-speed store. It’s usually connected via a special PCI or NuBus card that provides the necessary connections and works on one of several levels:-

Level 0

This requires at least two drives. Data is spread across a pair of the drives, a process known as data striping. Both drives act as a single volume, which is ideal for audio or video material at a fast data rate. However, all data is lost if any drive fails, so in effect this isn’t a redundant array. The array capacity is equal to the total capacity of all the drives.

Level 1

At least two drives are required, with data mirrored to a pair of master and slavedrives. This ensures high security and reasonable speed, since data can be read from either drive. The total capacity is half the capacity of all the drives.

Level 2

This requires at least three drives and a hardware RAID controller. Data is interleaved across several drives at bit level with Hamming code error correction. Unfortunately, if two or more drives fail it can be difficult to recover the data, which is why this level has been largely replaced by levels 4 and 5. Total capacity is 60% of the capacity of all the drives.

Level 3

Also requires a hardware RAID controller, but the data is sent to the paired drives whilst compressed data and parity information is sent to the third drive in each set. Since all the parity and error correction is on one drive it’s easier to recover data. Unfortunately, this single drive also creates a bottleneck that can restrict the system’s writing speed.

Level 4

Data is striped as blocks onto all three drives in a set whilst the parity and error correction information is sent to a third drive. Although better than level 2 and 3 it has similar limitations.

Level 5

Data is striped onto three drives with parity divided over all three, allowing hot-swapping of drives without a shutdown and automatic recovery of data from other drives should one fail. Drives in this type of system often have a Single Connector Attachment (SCA2) connection, allowing them to be inserted or extracted in a single action. This level is very fast, providing full redundancy and almost total data security. The total capacity is two-thirds of the capacity of all the drives.

Level 6

A complex cache system not used with Mac OS computers at the time of writing.

Level 7

A complex cache system not used with Mac OS computers at the time of writing.

RAID can be fitted to many machines by installing a PCI card, such as an ATA/133 RAID controller. Typically, this supports two or four drives and has links for selecting RAID 0 or RAID 1 levels. When using two drives you’ll normally need a power supply splitter cable and a separate data cable to each drive, both of which should have their links set to ‘master’.


MacWorld magazine (UK), IDG Communications, 2004

©Ray White 2004.