The magnetic surface in a hard disk has a rigid backing. It has 400 to 2,000
tracks per inch (tpi) with a data density of 7,000 to 60,000 bits per inch (bpi).
The disks spin at 3,600, 4,800, 5,400 or 7,200 revolutions per minute (rev/min) to
give a transfer rate of 5 to 30 M per second.
Standard Drives
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Main Features
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Standard hard disk drives use Winchester technology. The amount of data they can hold is given by the disk capacity. Typical values include 40, 80, 160, 240 or 500 M or more — this includes the space needed for formatting information.
The drive height may put constraints on where it can be installed as an internal drive or the enclosure that’s used for it. The standard heights are:-
Full-height 3.25 inch 84 mm
Half-height 1.625 inch 42 mm
Third-height 1 inch 28 mm
The disk diameter in a typical drive may be 1.8, 2.5, 3.5 or 5.25 inch. A smaller diameter makes for a faster drive but stretches the technology closer to its limits! A drive consists of one or more disk platters, each containing data, stacked one above the other and coupled together to form a single mechanism.
A large drive may use 5.25 inch diameter disks, or even as much as 8 or 14 inch! For drives over 1 G there may be up to 11 stacked platters, usually in a full-height package. Older 5.25 inch drives, originating from earlier Macs, are painfully slow.
A small drive, typically holding 40 to 220 M, often has 3.5 inch disks with 3 to 5 platters. PowerBooks use tiny 2.5 inch, 1.8 inch, 17 mm or 19 mm drives — their low mass makes them less vulnerable to mechanical shock and their small diameter makes them fast!
Mechanical Aspects
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The life of a drive is given by its Mean Time Between Failures (MTBF) — typically 50,000 hours or 5.7 years of continuous use. This assumes the drive is switched on and off twice a day. Such switching causes thermal shock that can shorten a drive’s life.
The drive heads must be withdrawn from the platters when not in use. This process, known as parking, protects both the head and the disk from possible damage caused by mechanical shock during transportation.
Most modern voice coil drives incorporate self-parking that automatically withdraws the head when the drive is unpowered. Older drives with stepper motors are not self-parking and rely on instruction from the Mac for parking. All Macs should automatically do this on shutdown, but this may not happen if the machine is simply turned off.
Î Drives must never be operated upside down — once used in this position they won’t
work the right way up! Drives may be used on their sides but shouldn’t be placed
above a power supply.
Î If a drive makes cricket noises try tapping or spinning the unit. This will loosen the
lubricant but the problem will come back! On some drives (where the spindle is not
obstructed by the logic card) it may be possible to apply additional lubrication.
Î It’s possible for the drive head to stick to the disk, so causing erratic behaviour. Once
again, a gentle thump may provide a temporary remedy. In the meantime backup
your data and contact an expert!
Speed
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It’s a surprising fact that many drives actually transfer data faster than than the microprocessors inside the original 680x0 series of Macs.
The speed of a drive is determined by its rotational speed, disk capacity, disk diameter, the head mechanism and the quality of driver software. Hence a good quality drive of small capacity and small size often out-performs a larger drive. If a drive has more platters the head can get to the data faster — especially if it’s a small size of disk!
Interfacing and Software
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Most Apple Macs use the Small Computer System Interface (SCSI) for external drive connections — 680x0 Macs also use it for internal connections. SCSI drives include a built-in disk drive controller and record at a data density of 36 sectors per track.
The alternative Integrated Device Electronics (IDE) or Enhanced IDE (EIDE) interface is used in PCs and also for PowerMac or Performa 630 internal drives — or in any computer that meets the PowerPC Platform (PPCP) standard. IDE drives also have their own drive controller but can be slightly slower than the SCSI variety.
External IDE or EIDE drives can’t be connected to most Macs — one exception is the PowerBook 5300 that incorporates both IDE and SCSI ports. It may also be possible to connect an IDE drive to a PowerMac if you can find a suitable PCI card.
√πSee the Parallel Ports & Slots chapter for more information about SCSI.
Î Most drives come with their own formatting and (hopefully) diagnostic software.
Always use the correct software for your drive!
Î Pre-formatted drives are preferable — if only to avoid the chore of formatting. But
remember — you’ll need the software to reformat the disk in the future!
Î Always check the suitability of both drive and software before using an enhanced
version of SCSI, such as SCSI-2 or SCSI-3.
Internal Drives
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An internal SCSI drive, for installation inside a Mac, has a 50-way header plug (for data) and a multi-pin plug (for power) fitted onto the actual drive card. The Macintosh itself may have a 50-way ribbon cable already installed to accommodate an extra drive but in some cases you may need to connect an additional ribbon cable to the motherboard.
If there are insufficient sockets on the motherboard you may need to use a 50-way splitter cable to provide the extra connection. Most machines require a power connection cable to join the extra drive to the motherboard. Once again, if there aren’t enough connectors on the motherboard, you’ll need a splitter cable.
Î Take care connecting the 50-way ribbon plug to your drive — it can go in backwards.
One end of the connector is marked with a dot or triangle which corresponds with
that on the drive. The ribbon cable itself may also be marked on one edge by a colour
— but this can’t always be relied on! If in doubt, consult an expert.
Î The power connection cable for most Macs are standardised — but earlier machines
used a different type of connector at the motherboard end. Your supplier should be
able to provide a suitable alternative cable.
Special links on the card are used to set the SCSI ID number, SCSI termination and various other options. Don’t tamper with these links unless you know what you’re doing!
External Drives
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An external SCSI drive is really an internal drive and power supply fitted into an enclosure. External SCSI connections are often provided via one or two 50-way Amphenol sockets. Some earlier devices use 25-way D connectors to match those on Compact machines or a flying lead that plugs directly into the back of a Mac.
SCSI termination and SCSI ID number may be set by switches on the rear panel or by links on the drive card — in the same way as an internal drive.
Removable Disks
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  Removable disks combine the speed and flexibility of a hard drive with the
portability of diskettes. They’re particularly useful for backups or for use as an
emergency startup disk.
Hard Disk Cartridges
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Cartridges use the same technology as in fixed drives and often incorporate SyQuest, Bernoulli, or Ricoh mechanisms. To avoid data corruption you should always handle cartridges with care and avoid dust. Larger drives can usually read smaller capacity disks of the same type but you should check this before buying!
5.25 inch (SyQuest)
Well-proven technology accepted as a standard by many, even if it is bulky! 44 M, 88 M and 200 M versions are available.
3.5 inch (SyQuest)
An early attempt to produce a convenient 3.5 inch format — the original 105 M and
270 M sizes are now supplemented by a 540 M version,
3.5 inch EZ-Series (SyQuest)
A recent low-cost format, beginning with the highly successful EZ135, storing 135 M and now expanded to 230 M in the EZFlyer. These disks are incompatible with the earlier version above.
3.5 inch Zip (Iomega)
An innovative device that uses what looks like a thick floppy disk to store 100 M.
3.5 inch Jaz (Iomega)
Another proprietary format — but this has a capacity of 1 G. A drive of this size is essential if you need to store large graphics files.
Other Formats
Several proprietary formats have come and gone over the years — anyone who buys a ‘pioneering’ product should take note! The Bernoulli (Iomega) 44 M and Ricoh 50 M cartridges have been displaced by SyQuest products. Similarly, a large 10 M floppy disk by Verbatim seems to have disappeared without trace!
Formatting Hard Disks
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Formatting software prepares your drive prior to the installation of software.
It’s supplied by the disk manufacturer or a third party.
Some formatting software is of doubtful origin! You should always use software that matches the drive — check first to see if the software recognises the drive correctly!
Although you may persuade an unsuitable program to format your drive it won’t install the correct driver (see below) and the life of the drive may be compromised.
° Before formatting you must restart your Mac with all extensions turned off
Î Formatting software may not work if a RAM Cache is enabled.
Î Always use Apple HD SC Setup for an Apple SCSI drive. Use Drive Setup for a PowerMac
or any other Mac with an Apple IDE drive. The latter can format an unrecognisable
drive, but not if it’s a removable disk!
Î Almost any SCSI drive can be formatted with HDT Primer (part of HD Toolkit),
SpotOn or Silverlining.
Low Level or Physical Formatting
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° Low level formatting destroys all the data on a drive
The formatting process organises useable sectors in each circular track or cylinder of the disk into packages called data blocks. The largest possible block could occupy an entire cylinder whilst the smallest possible block would be equal one sector.
A number of alternate or spare sectors are assigned to replace any faulty sectors — there’s usually one spare available in every track. Details about these bad data blocks are stored an area of RAM assigned to the drive. The Mac allocates these spare sectors as necessary
Î It’s advisable to format a drive every 2 or 3 years to compensate for mechanical drift.
Such problems can cause System crashes.
Block Size
Block size is often selected automatically by the formatting application. It’s invariably related to the drive size or partition size (see below). For example, a typical 150 M drive uses 2.5 K blocks, a 240 M drive uses 4 K and a 600 M drive uses 9.5 K blocks.
If your application lets you choose your own block size you should remember that even the smallest file occupies an entire block. For example, with a block size of 4 K, a 500 byte file will always use 4 K of disk space. If you have lots of small files this wastes disk space — if it’s a problem the drive can be split into partitions that use suitably smaller blocks.
Interleave
Sectors may be interleaved to help the processor read the data. Older Macs used a 2:1 or 3:1 interleave whilst the Apple II used 4:1. Recent machines use 1:1 — in other words, no interleave at all!
A drive works faster if the interleave preferred by the machine matches that used to format the disk. For example, a drive initialised on a Mac Plus and then used with a new Performa won’t be as fast as one formatted on the Performa itself. The Performa will have to wait for every second or third block of data, rather than using the blocks in sequence.
If a drive’s formatted on a Performa and used with a Mac Plus, it’s even worse! The blocks are recorded in sequence but the Plus can’t accept data that fast. Hence the disk has to turn almost a full rotation before the next block again passes the head, and the data received.
Blind Writes
This is an option provided in more advanced formatting applications. With blind writes selected the Mac doesn’t check any of the data that it records onto a drive. This often doubles the writing speed but at a serious risk to data integrity. For that reason most formatting applications leave blind writes turned off by default!
High Level or Logical Formatting
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In Mac terminology this is called Erasing. The process rebuilds the Directory, assembles the Boot Blocks (defining the beginning of a Mac disk, memory setup and Startup files), creates the Volume Information Block (defining which folder the System and Finder are in) and makes the Volume Bit Map (allocating where data is to be stored on the disk).
Î Erasing destroys the data which identifies old files but the files themselves remain,
unless overwritten by new material.
Î All drives, whether made by Apple or not, can be erased under the Special menu —
but you can also use the drive’s formatting software.
Disk Drivers
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Disk driver software translates the needs of the system into commands that can be understood by the SCSI disk controller inside the drive. When a drive is formatted this software is recorded onto the disk itself. When the drive is mounted (see below) the disk driver is transferred into the RAM assigned to it.
Î You should always update the drivers on a disk using Apple HD SC Setup or
Drive Setup before updating the system on that drive.
Î Different drivers can exist in their own partitions on a drive (see below). Software
such as HDT Primer can activate selected drivers or remove unwanted ones.
Drive Setup has drivers for ATA (IDE), Avid, Conner, Fujitsu, IBM, Iomega, Must, Quantum, Ricoh, Seagate, Sharp, Sony, SyQuest, Apple SCSI HD and PowerBook drives.
Before updating the system on an IDE drive you must use the latest edition of Drive Setup to update its ATA drivers. If Update Driver, Customise Volume and Test Disk are dimmed you must backup all the data and initialise the drive from scratch.
Mounting
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When a disk is mounted its icon appears on the desktop. Disks should auto-mount whenever a removable disk is inserted. A disk inserted prior to startup should always mount — the system simply reads the driver software from the disk. If you then remove the disk and re-insert it the icon should re-appear, but only if the driver is kept in RAM.
The real problem occurs when a disk is inserted into a drive which was empty at startup — in this case the driver hasn’t ever been in RAM! To overcome this you’ll need a SCSI extension or control panel, usually supplied with the drive’s formatting software. Place it in the appropriate folder in the System Folder and then restart your Mac.
Such software will only auto-mount a drive if it recognises the driver software. For example, SpotOn cdev (a control panel) can auto-mount any disk formatted with SpotOn — but not one formatted with HDT Primer. There are two solutions — you can format all your disks with one formatting application or you can install two SCSI extensions or control panels.
If a disk still fails to mount try the following until the icon appears:-
~ Update and Mount — using SCSIProbe or HDT Prober
~ Mount — using the formatting software
~ Reinstall drivers — data may be damaged in the process
~ Initialise — the disk will be erased
~ Format — the disk will be erased
Î Some SCSI extensions or control panels can conflict with each other!
Initialising
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This removes drivers, partitions and partition maps. It then installs new drivers and/or partitions. With some software this is preceded by formatting.
Î When using Apple HD SC Setup the Update box may be greyed out — this indicates
you have a non-Apple drive. If this happens you should contact the supplier of the
drive or use a universal formatting application such as SpotOn or HDT Primer.
Î Initialisation in Disk Setup can perform a low-level format (see above) or simply set
all data bits to zero. Initialisation Options let you erase all data before initialising.
Partitioning
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Partitioning divides a disk into watertight compartments, called logical volumes, that can be used for different purposes.
At least one partition (the largest) is set aside for the Mac’s System. Don’t be confused by this — this partition is the part of the drive in which you store files. The term ‘System’ simply refers to the fact that they operate under the Mac’s system! Other partitions are given to the Hierarchical Filing System (HFS), driver(s) software and optional free space.
You can add extra partitions for your own convenience — or for subdividing a large SCSI drive where each partition must be less than 1 G in size. In an ideal world you would divide your drive into at least three System partitions — the first set of tracks for the System Folder, the next for applications and the last for data.
Î Track zero (the first track) of a disk is located at the outermost edge of the disk.
Î If you use PC Exchange you can have any number of Macintosh, MS-DOS or ProDOS
partitions on a single drive or removable cartridge. Any ProDOS partition, used for
data from an Apple II computer, must be less than 32 M in size.
Hard Partitioning
These partitions are created during low level formatting by dividing the disk into groups of cylinders (tracks). It’s impossible to change these partitions after formatting.
Soft Partitioning
Formatting software such as HDT Primer or Norton Utilities lets you define your own partitions, after initialisation. The drive then appears as two or more icons on the Desktop. The partitions can be altered at a later date — but always backup the data first!
Î In Disk Setup you must use Customise Volume to create partitions. You can have up to
eight HFS partitions — or one or two ProDOS partitions plus a single HFS partition.
Following this you should test the disk and choose whether it should Automount on
Startup or should be write protected.
Verifying
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This is a useful check on the reliability of a drive and is often provided as part of the formatting process. Two methods are used:-
z Full verification or Test
The entire surface of the disk is checked by recording on it — all data is lost. This
process identifies faulty sectors which will skipped on subsequent operations.
z CRC (Cyclic Redundancy Check) Verification
This is a non-destructive process whereby the data in each sector is compared with
related redundant values located at the start of each sector. The process doesn’t
harm data but may identify faulty sectors, making them inaccessible.
The most common system is CRC-16 in which data is split into blocks of 128 bits
(16 bytes). A single bit is then chosen from each byte, in a predefined sequence, to
create the CRC block itself. These redundant bits can then be used to check the
validity of the data. This technique is not foolproof — two wrongs can make a
right! Full verification, which also confirms CRC values, is the acid test.
Hard Disk Solutions
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Disk Fragmentation
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In a fragmented disk the data for each file gets scattered across the disk and interspersed with other material. This slows done the drive as it hunts for all the pieces to make up the files. The drive must then be defragmented (optimised) using an application such as Speed Disk, CP Optimiser and DiskExpress.
Optimisation checks for weak sectors and then marks these bad blocks to avoid them in future. It then extracts files from the disk and writes them back into contiguous sectors on the same drive — each file then occupies less disk area, thus reducing wear and tear on the mechanism. A small amount of disk capacity may be lost in the process.
° Always verify the disk and backup your data before optimisation!
Î Optimise a drive when fragmentation reaches 5-10% — usually about once a month.
Î Some software installers put applications at an absolute sector address to protect
them from being copied. Optimisation MUST NOT be used on partitions that
contain such applications.
Î You can minimise the effects of fragmentation by splitting a drive into two or more
partitions (see above) — then you may only need to optimise a particular partition.
Other Hints
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Î The speed of a drive is measured by checking its seek time — the time taken to reach
a given sector. Some formatting packages, such as HDT Toolkit, can do this test.
Î Apple’s HD SC Setup won’t recognise a drive that it hasn’t formatted — so you
can’t use it’s Test facility either!
Î If your using Norton Utilities on an IDE drive you should check it’s suitability first!
Î Files consigned to Trash, which is then emptied, are not lost (only the identification
has been removed) and may be recovered using software such as Norton Utilities.
But if another file is written to the disk your files may be lost forever.
Î Disks that are unrecognisable to a Mac may not need reformatting. The main cause of
disk failure is corruption of the data which identifies the type and size of drive.
Without this information the computer can’t recognise or mount the disk. In most
cases this can be recovered by using the Disk First Aid application supplied with
the System Installer. Just select the drive and follow the instructions.
Î If you want to avoid data corruption don’t switch off your Mac whilst there’s disk
activity — check for this with the disk activity light or by listening to the drive.
Î If you have a volume of more than 2 G it’s Get Info dialog can show a negative
number of bytes — this doesn’t indicate a problem!
Special Disk Systems
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Removable Drives
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A removable drive shouldn’t be confused with a cartridge. With this device the entire drive, complete with heads, is removed! Sizes range from a 3.5 inch 425 M drive to 1.6 G or more. They provide a high level of data security (you can put a drive in the safe!) and are unlikely to suffer from data corruption as a result of dust — but they’re highly expensive. These drives have almost been superseded by magneto-optical disks.
Redundant Array of Independent Disks
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RAID uses a number of drives to provide a large, high-speed data store. It needs a special NuBus or PCI card in a Mac and can operates on one of several levels:-
0: The data is spread across a pair of disk drives via two SCSI ports as provided on
the Quadra 900 and others. This can transfer data at up to 5.5 M per second but all
data is lost even if only one drive fails.
1: Mirroring is used to send data to both drives to prevent loss of material.
2: Not used by Macintosh.
3: A complex technique which records data on two drives and also records
compressed data plus parity information to a third. Drives may be disconnected
without closing down the system.
4: Not used by Macintosh.
5: This method records data on all three drives together with parity which is divided
over all three. Drives may be disconnected without closing down the system.
6: Complex cache system — not yet used by Macintosh.
7: Complex cache system — not yet used by Macintosh.
Non-Macintosh Drives
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There are a range of hard disk drives that can’t be connected directly to a Mac. Most of these have a special serial interface and require external hard disk controller electronics, unlike SCSI, IDE and EIDE drives that have a controller built into the drive.
The earliest form is the Modified Frequency Modulation (MFM) drive, typically with 17 sectors per track and 512 bytes per sector. Data transfer rate is usually around 5 M per second and the drives can hold a 100 M or more. A pair of MFM drives can be connected to a single controller card using two 20 way ribbon cables for data and a 34 way ribbon, parallel-connected to both drives, for control purposes.
Run Length Limited (RLL) drives use a variant of MFM that increases data density by 50%, with 26 sectors per track and a transfer rate of around 7.5 M per second. Enhanced Small Device Interface (ESDI) drives use a double-density version of MFM, employing 34 sectors per track to give rates of 10 to 15 M per second, similar to EIDE drives.
Tape Drives
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  Tape drives are an alternative to hard disk for archiving purposes.
Although slow they are reliable and use low-cost media.
A tape drive is usually connected to a Mac via the SCSI port. The device doesn’t usually appear as an icon on the desktop — you would have to wait a long time to open a file! However, DeskTape software can provide an icon if you really need it.
The traditional quarter-inch cartridge (QIC) can store between 20 and 160 M, expanding to 250 M or more with data compression. Typical sizes include 150 M, 250 M, 525 M, 1.2 G, 2.0 G and 2.5 G. The mini-cartridge version can store 40 or 120 M.
An 8 mm cassette, as used in helical scan video recorders, can store 3.5 G (uncompressed) or 7 G (compressed) on a 160 m tape or 2.5/5 G on a 112 m tape. It can take several minutes to reach a particular location on the tape. Different data formats may be used by different devices, although Exabyte is a common standard.
A 4 mm cassette, as used in Digital Audio Tape (DAT) recording, stores 2 to 16 G, depending on tape length. Access to any part of the tape usually takes around 20 seconds. Different data formats are used by different devices — DDS/DAT, developed by Hewlett-Packard and Sony is common, although DATA/DAT can also be used. The DDS-2 format can store 4 G (uncompressed) or 14 G (compressed) on a 120 m tape. Without compression a 60 m tape stores 1.3 G and a 90 m tape holds 2 G.
The following table shows the performance of a typical drive:-
Capacity (G) Transfer Rate (M per minute) Tape Size (metre)
