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Hard Disk Mechanics

Hard disks are fantastic pieces of equipment relying on magnetic storage technology. Today's standard hard disk has a 3.5 inch wide (the same width as a floppy drive) casing which encapsulates the assembly itself. The image below shows a hard disk with the casing removed:

The disk is assembled as follows: flat circular disks called platters coated with a magnetic media are stacked onto a spindle, much like a vertically stacked pile of disks in a jukebox. Traditionally, platters were made of aluminium. Glass is becoming ever more popular as the surface of a glass platter is much smoother. The spindle motor rotates the platters are a tremendous speed, typically 5400 or 7200 revolutions per minutes in modern drives; this is called the spindle speed. Hovering above each platter on a cushion of air, at a distance of approximately 2mm, is an electromagnetic read/write head mounted on the end of an actuator arm.

As an aside: the air cushion is actually generated as the drive spins. When stationary, the heads actually contact the platter. However, the heads do not 'fly-off' the platter until the spindle speed reaches a critical level, much like an aircraft needs a particular speed to lift off. So, while the drive spins up to speed, the heads are in contact with the platters, 'scraping' along the surface. To prevent loss of data while spinnng up to speed and when spinning down, the drive heads are pulled to a 'safe-zone' called the landing zone where no data is held. In the old days, there were utilities to carry out this head-parking before you shutdown your PC. Modern drives do it for you, so you don't have to worry about it.

Each platter typically has a second read/write head (on its own arm) sitting below it. This is because data is stored on both surfaces of a platter. These arms are also arranged into a stack, called a block. A slider on each actuator arm moves its head along the radius of the platter. Each platter with its associated pair of actuator arms and heads is much like a record turntable and needle (where the position of the needle above the record is similarly controlled by an arm), except in our analogous turntable there would be a needle on both sides of the record. Hard disks are serial access devices, meaning that the heads wait for the platters to pass under them. This is very different to random access where any location can be selected at will, without having to wait for the data to be presented in turn. For more on this topic, check out the memory section.

A circuit board on the base of the drive coordinates the activity of the mechanical components and communicates with the system.

Now move on to look at the popular types of hard disk interface.