The hard disk is made up of a collection of disks known as platters. It is sealed in a vacuum container, and is located inside the system unit. When the tracks having the same track number on the different platters of a hard disk are joined together, they form a cylinder.
The read-write mechanism in a hard disk is slightly different from that of a floppy disk. Here, the read-write arm (containing twice as many read-write heads as there are platters, since each platter has two recording surfaces) is positioned over the recording surface and the disk is rotated. The read-write head then picks up the necessary data, which is transported to the RAM through the data bus. The hard disk controller performs functions similar to that of a floppy disk controller.