Hard disks

A stack of one or more metal platters (usually aluminium) coated with iron oxide that spin on one spindle.
Typical capacity(1998 Aug): 2 - 6GB

Types;

Exchangable - may be removed from the PC (eg. Syquest or JAZ drive). Note that the ZIP disk is actually a floppy disk based exchangable disk drive.
Fixed Disk. Sealed unit, with built in read/write heads. Remains with the system unit.
Optical - require a laser to read/write the data.

Winchester Drive:
Hard disks used to be called Winchester drives: This term relates back to the 1960's when IBM developed a high-speed hard disk drive with a 30MB fixed platter and 30 MB removable platter. The 30-30 drive was soon nicknamed the Winchester after the Winchester 30-30 rifle. Up until the mid 80's a high speed spinning platter with floating heads was referred to as a Winchester drive.

Features

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Sealed unit
Built in Read Write Heads
Reliable
usually multiple platters, and sides per platter
Physical : 3.5 to 5.25 inch diameter, Half Height, or Full Height or third height.
Hard Cards are also an option fitting into an expansion port.
For access times, data transfer, storage etc, link to Comparisons

Physical Characteristics

Hard Disks may be;

Kept in a sealed unit and mounted internally in the system unit. (Winchester Drive)
Attached to a circuit board and inserted into a computers expansion slot (Hard Card)
Kept in a sealed unit and mounted externally from the system unit. (eg. Backpack Drive - attached via a parallel port, or a SCSI drive)
Stored in a disk pack, or disk cartridge and mounted on-line when required

Read/write heads

To enable high speed access the Heads 'float' over the disk surface.

The diagram illustrates the distances involved, and compares them to other common particles.

Cylinder

For a Hard disk multiple platters are stacked one on top of the other.
Multiple Read/Write heads are linked together and write to the disk, to what is commonly referred to as a cylinder.

Exercise 1: Cylinders, Sectors, and Tracks

Show how you may derive the total number of Bytes on a 360 Kb Diskette if the Diskette has the following properties ;

40 Tracks per Side
9 Sectors per Track
512 Bytes per Sector
2 Sides per Disk

Sample Answer

Logical Characteristics

When hard disks were first introduced, MS-DOS allowed disk storage sizes of no more than 32 MBytes.
As hard disks became larger, this limit had to be circumvented.

This was achieved by allowing logical Partitions to be created.

This is where a single physical hard disk may be logically divided into multiple drives.

Today however DOS (and other Operating Systems) allow for drive capacity to exceed 32Mbytes, but the concept of a Partition has remained.

Exercise 2

Explain how partitioning may be used on a Hard Disk

Sample Answers

Standards

There are four standard Interfaces for Hard Disks and their controllers, on PC's.

Modified Frequency Modulation (MFM)

Once popular, drives tend to be slower
Controllers use expansion cards
May not work well with > 100Mb drives

Enhanced Small Device Interface (ESDI)

designed for very large drives (Not common in the 100 Mb range)

Imbedded Drive Electronics (IDE)

Controller on Drive, therefore easy to connect
PS/2 and many PC's put the connector on the motherboard.

Connection available via inexpensive adaptor cards
Normally insist on being the primary drive
Relatively inexpensive

Small Computer System Interface (SCSI)

Controls up to seven devices (including CD-ROM, scanners and some printers), each with it's own individual controller.
Not all SCSI devices work with all SCSI adaptors. SCSI 2 standards developed to hopefully (!) give greater compatibility.

Run Length Limited

Some drives and controllers use RLL (Run Length Limited) encoding to reach 100Mb. You should ensure that both drive and controller support RLL, or you'll end up with less capacity, or if an RLL controller is used with a non-RLL drive data loss will be likely. (not possible with IDE or SCSI as controller with the drive)
If you are adding larger drives to existing PC's the ROM BIOS may need modifications to support the extra capacity.

Ref [1]

Removable or Exchangeable

Modern

Advantages

Unlimited, transportable storage

Features

Magnetic media has a half life of 5-7 years (the magnetic charge weakens to half its original strength).
Drop height (distance from the floor you can drop the cartridge without making it unplayable).

Examples

Iomega Jaz:

Capacity: 1GB (1996) or 2GB (1998)
Platter rotation: 5,400rpm
90cm drop height (since it is a hard disk)
Access time: 15.5ms read/ 17.5ms write
Average seek time: 10ms read/12ms write
Max sustained transfer rate: 6.62 MB/sec max.
Average data transfer rate 5.4MBps (1GB) and 7.4MBps (2GB)
Backup time 20 minutes (2GB)
Costs: (1998) NZ$200 (1GB), NZ$300 (2GB).

(Removable Feast: Jaz 2GB Drive, Apr 98)
(Knorr E., Apr 96) & Zip drive 100 brochure.
Also refer to Zip Drive

Syquest EZ135

Formatted: 126MB
Originally designed for Apple Macintosh.
40% faster than Iomega Zip (Twice as heavy)
Drop height 1m (Hard disk based)
SCSI connection
Futures: 1.3GB drive expected!
Price:(Apr '96) Drive $535 cartridges $38.

(Knorr E., Apr 96)

Traditional

Hard Disks may also be removable, as in the case of a Disk Cartridge.
Disk Packs were commonly used by mini-computers or mainframe computers, for large capacity data that was not required permanently on-line.

Sample Answers

Answer 1: Cylinders, Sectors and Tracks

(512 Bytes x 9 sectors x 40 Tracks x 2 sides) = ?

Answers 2: Logical Partitioning

Separation of Programs and data
Separation of System Software (eg. OS/2), Application Software, and Data.
Allows separation of multiple operating systems on One Hard Disk
(IBM OS/2 Boot manager)
Separation of
static programs and data - such as the Operating system and application programs, and
changeable data - such as your WP or Spreadsheet documents. This may then be defragmented as and when required.
If you wish to use disk compression it is often recommended that you create two partitions. The secondary partition is then compressed, leaving the primary partition in its native form.
If you are using a Video capture board to capture video directly to the hard disk, a small partition that is easily re-formatted will provide maximum frame capture rate.