Read/write head of a hard disk. Data is recorded on to a computer hard disk by a read-write head on the tip of a metal arm (centre). The arm moves as often as 50 times per second over the spinning magnetised disk or platter without touching it. The read/write head magnetizes and demagnetizes domains on the platter, thereby storing and retrieving bytes of information. Due to the speed of the read/write head, data can be accessed from the hard disk in a fraction of a second. Most hard drives have more than one platter and several read/write heads to increase the amount of information that can be stored.