Despite the phenomenal increase in mobile devices such as mobile phones and PDAs over recent years, the hard disk drive (or just "hard disk" or "hard drive") remains the most common focus of computer forensic investigation. In this article we will take a look at the components which make up a typical hard disk drive and how they work together to allow data to be stored and retrieved.

In many ways, the basic design of the hard disk has changed very little since its introduction, although changes to individual components have brought about remarkable improvements in speed, capacity and reliability. Let's open up a disk and see what's inside...

Exposing the internals of a typical hard disk immediately reveals the two major components: the platters and the actuator arm.

Platters

Platters are the circular disks located one above the other and mounted on a central pole known as a spindle. The platters are specially coated on each side with a coating which enables them to store data in magnetic form. Data is stored in concentric circles on both upper and lower surfaces of the platter and these circles are referred to as tracks. Each track is divided into individual sections called sectors and we will see later how sectors and tracks are used to locate specific areas of the disk. When a disk is powered on and the system needs to read or write data the platters spin at a very high speed (driven by the spindle motor) which enables the correct part of the disk to be read through use of the actuator arm and the components associated with it which are described next.

Actuator arm (or actuator assembly or head assembly)

In order to actually read data from or write data to a disk (i.e. to or from a platter) a tiny device called a read/write head needs to be positioned just above the platter surface. In order to position the head correctly it is located on the end of a "head arm" (strictly speaking, the read/write head is attached to a "head slider" which itself is attached to the head arm). The number of head arms present in a hard disk is determined by the number of platters with each arm usually being used to position a read/write head on either side of a platter. Head arms are joined together in a structure referred to as an actuator arm, actuator assembly or head assembly. In order to position the read/write heads in the correct place on the platters the assembly pivots on an axis which allows the heads to move across the platter surface. This action, together with the rotation of the platter itself, allows the heads to be positioned correctly.