Hard Disk Drive

On almost all computers, the hard disk drive is by far the most important storage device. A hard disk drive (or simply hard disk) is a high-capacity, high-speed storage device that usually consists of several fixed, rapidly rotating disks called platters. The computer’s hard disk also referred to as online storage. Online storage (also called primary storage) consists of the storage devices that are actively available to the computer system and that do not require any action on the part of the user. Hard disks can also be categorized as random access or magnetic storage device. A random access storage device can go directly to the requested data without having to go through a linear search sequence. Magnetic storage devices use disks that are coated with magnetically sensitive material. With magnetic storage devices, an electromagnetic called a read/write head moves across the surface of a disk and records information by transforming electrical impulses into a varying magnetic field. As the magnetic material pass beneath the read/write head, this varying field forces the particles to rearrange themselves in a meaningful pattern of positive and negative magnetic indicators. This action is called writing. When reading, the read/write head senses the recorded pattern and transforms this pattern into electrical impulses. A hard disk contains two or more vertically stacked platters, each with, two read/write heads (one for each side of the disk). The platters spin so rapidly that the read/write head floats on a thin cushion of air, at a distance 300 times smaller than the width of a human hair. To protect the platter’s surface, hard disks are enclosed in a sealed container. Two or more sector combines to form a cluster.