regarding Hard Disk

A hard disk drive (HDD, also known as hard disk, hard drive, or the now-near-obsolete terms fixed disk, fixed drive, fixed disk drive, hard file) is a digitally encoded non-volatile storage device which stores data on the magnetic surfaces of hard disk platters. Hard disks were originally developed for use in connection with, or later inside, a single computer. Later, as a way of guarding against hard disk failure, they were arranged into configurations such as redundant array of independent disks (RAID). Hard disks are also found in network attached storage (NAS) devices, but for large volumes of data may be most efficiently used in a storage area network (SAN). Over time, applications for hard disk drives have expanded beyond computers to include video recorders, audio players, digital organizers, and digital cameras. In 2005 the first cellular telephones to include hard disk drives were introduced by Samsung and Nokia. Hard drives record information by magnetizing a magnetic material and read the data back by detecting the magnetization of the material. A typical hard disk drive design consists of a spindle which holds one or more flat circular disks called platters, on to which the data is recorded. The platters are made from a non-magnetic material, usually glass or aluminum, and are coated with a thin layer of magnetic material. Older drives used iron(III) oxide as the magnetic material, but current drives use a cobalt-based alloy. The platter's magnetic surface is divided into many small regions, each of which is magnetized independently of the others. The platters spin at high speeds. Information is written to a platter as it rotates past mechanisms called read-and-write heads that fly very close over the magnetic surface. The read-and-write head is used to detect and modify the magnetization of the material immediately under it. There is one head for each magnetic platter surface on the spindle, mounted on a common arm. An actuator arm moves the heads on an arc (roughly radially) across the platters as they spin, allowing each head to access almost the entire surface of the platter. Source: http://www.answers.com/hard+disk? initiator=IE7:SearchBox