Hard disk platter

Close-up of a hard disk head resting on the disk platter. The smooth disk surface shows a mirror image of the head/slider resting on its surface.

A hard disk platter (or disk) is a component of a hard disk drive: it is the circular disk on which the magnetic data are stored. The rigid nature of the platters in a hard drive is what gives them their name (as opposed to the flexible materials which are used to make floppy disks). Hard drives typically have several platters which are mounted on the same spindle.

The magnetic surface of each platter is divided into small sub-micrometre-sized magnetic regions, each of which is used to represent a single binary unit of information. A typical magnetic region on a hard disk platter (in 2006) is about 200-250 nanometers wide (in the radial direction of the platter) and extends about 25-30 nanometers in the down-track direction (the circumferential direction on the platter), corresponding to about 100 billion bits (1 gibibit) per square inch of disk area. The material of the main magnetic medium layer is usually a cobalt-based alloy. In today's hard drives each of these magnetic regions is composed of a few hundred magnetic grains, which are the base material that gets magnetized. However, future hard drives may use different systems to create the magnetic regions. As a whole, each magnetic region will have a magnetization.