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How A Hard Drive Works

General

Basically, a hard drive is constructed of a sealed metal housing, magnetically coated platters, a group of read/write heads and an electric motor. Together, these elements are used to both read and write data to the surface of a platter, which is the basic means of storage for many computers.

The Hard Drive is controlled by both a logic board and a controller card. The hard drive receives commands from the computer to read and write data to the platters and the commands are translated into mechanical movement. When the computer requests that a string of data is written to a hard drive, the logic board instructs the write heads to align the magnetic field of particles that are located on the platters surface resulting in data being written to the hard drive. The data can be retrieved later by requesting that the read heads detect the polarities of the aligned particles. The process of reading and writing files is directed through a hard drive's file allocation table (FAT) which determines the physical starting location of a file, and records chains of data that describe the location of all files on the platters. The FAT keeps track of available space as well as space already in use and is also referred to when the system attempts to write new files to a platter.

Platters

The hard drive can have one to several disks made of glass or aluminum coated with magnetic iron oxide particles. These disks spin continuously very fast and the head can travel in and out along a radius so that any location can be reached very quickly by the head. A typical hard drive contains several of these 3.5-inch platters, which can contain tens of billions of individual bits. The higher the density of the hard disk's platters, the more bits that can be packed into each square inch of the platter surface. A platter is segregated into tens of thousands of concentric tracks. Because huge amount of information can be stored in one track, the tracks are broken down into smaller units called sectors. Disk platters are mounted in a stacked formation on a spindle A spindle motor turns the platters at very high speed, typically either 5,400 or 7,200 rotations per minute (RPM), but some are as fast as 15,000 RPM. The platters spin so that the appropriate sector or sectors containing the data can be positioned underneath one of the drive's reading heads. There's one head per platter, and all the heads move in unison.

Heads

Each head in the hard drive is mounted on a slider, which is mounted on an arm. A mechanical device called an actuator controls each hard drive arm. The actuator moves the arm to the correct position on the spinning platter. The reading head ( reading and writing heads are separate) floats about 2/1,000,000 of an inch above the disk surface. It interprets the magnetic pulses and converts them to electrical pulses that can be interpreted as ones and zeros as it passes over the appropriate disk sectors. Although the head may look large the actual exposed portion of the head, which can either read or write the information on the disk, is very small.

Improvements

As the manufacturing technology has improved over the years manufacturers have been able to shrink the exposed portion of the head down to smaller and smaller sizes. That is how the hard disk memories have risen to such large figures. Mmanufacturers have also increased the density of magnetic particles on the disk thereby making the storage capacity larger.