The Hard disks

Hard disks

Hard disks consist of one or more magnetic disks contained in a box. They are used as storage media in the PC, where you store programs and other digital data.
The magnetic storage hard disk is based on a 40 year old technology. It has been and still is being improved rapidly. Hard disks continue to shrink in size, gain increased storage capacity and increased transfer speeds. The development has been tremendous during the last 10 years. Indications are that this will continue for a long time.

When buying a PC, It is a good rule to include a large and fast hard disk. You can never buy too large a hard disk and the data transfer speed is decisive for the PC's performance.
An evaluation of the hard disk, its configuration and performance, involves several different technologies.
That is the subject of this page:
  • The mechanical disk. The physical disk construction, RPM, read/write head, data density, etc.
  • The cache. The hard disk always has some cache RAM onboard. It serves as a buffer, so the data being physically read is best utilized.
  • The interface. The connection between the hard disk and other PC components. That is called interface - the connection to a data bus, the controller principle.
  • Formatting etc. Disk formatting, control system, cache, etc. I presume you are running in Windows 95, which has the best access to the hard disk.
I want to illustrate the inter-action between these features, thus giving a comprehensive picture of the hard
disk and its technologies.


First, let us look at the hard disk history. IBM introduced the first hard disk in 1957. That was a major project. It consisted of 50 platters, 24 inch diameter, with a capacity of 5 MB, a huge storage media for its time. It cost $35,000 annually in leasing fees (IBM would not sell it outright). The first model to use "float on air" technology for the read/write heads was named Winchester 3030. So named because it was developed in Winchester, England and had two sides, each of which could store 30 MB. To some people, this designation was reminiscent of the famous Winchester 3030 repeating rifle. Later, the disk platters shrunk to 14" and 8" diameter. They were installed in towers containing dozens of these magnetic platters.

In the early years of PC development, the low cost floppy drives were the preferred storage media. But with IBM's XT in 1983-84, the hard disk became the preferred media. The first hard disks were rather large units (5.25" diameter) and of poor quality. I have replaced numerous 5, 10 and 20 MB hard disks during 1986-88, since these early PC hard disks had an incredible short life span. Since then they have improved a lot.

The modern hard disks are 3.5" diameter. A typical example is the Quantum Fireball, which you see above. The cover plate has been removed, so you can see the top arm with its read/write head.

Hard disks can be found in much smaller sizes (all the way down to match box size). However, for ordinary, stationary PC's the 3.5" is the best. They are inexpensive to manufacture, and they are faster.

Physical aspects of the hard disk
First, let us look at the construction of the hard disk.

Read/write heads

All hard disks consist of thin platters with a magnetic coating. They rotate quite fast inside a metal container. Their design causes them to ride on a microscopic cushion of air, without touching the platter. Data are written and read by read/write heads. They register bits from the magnetic coating, which races past them. On the illustration below, you see a hard disk with three platters. It has 6 read/write heads, which move synchronously.
The arms, which guide the movement of the read/write heads, move in and out almost like the pick-up arm in an old fashioned phonograph. As illustrated below, there will typically be 6 arms, each with read/write heads. The synchronous movement of the these arms is performed by an electro-mechanical system called head actuator. The hard disk data can only be attained via one head at a time.

The read/write head consists of a tiny electromagnet. The shape of the head end acts like an air foil, lifting the read/write read slightly above the spinning disk below.

When the disk rotates under the read/write head, it can either read existing data or write new ones
  • If a current is applied to the coil, the head will become magnetic. This magnetism will orient the micro magnets in the track. This is write mode
  • If the head moves along the track without current applied to the coil, it will sense the micro magnets in the track. This magnetism will induce a current in the coil. These flashes of current represent the data on the disk. This is read mode.
The read/write heads are by far the most expensive parts of the hard disk. They are incredibly tiny. In
modern hard disks they float between 5 and 12 micro inches (millionths of an inch) above the disk. When the PC is shut down, they are auto parked on a designated area of the disk, so they will not be damaged during transport.