"The hard drive can have one to several disks made of glass or aluminum coated with magnetic iron oxide particles. The disk(s) spins continuously very fast in the hda 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 Ariel density of the hard disk's platters, the more bits that can be packed into each square inch of platter real estate. A platter is segregated into tens of thousands of concentric tracks. Because allot of information can be stored in one track, the tracks are broken down into smaller units called sectors. Each sector can hold about 512 bytes of data, or 4096 bits. Disk platters are mounted in a stacked formation on a spindle A spindle motor turns the platters at very high speed, typically between 5,400 and 7,200 rotations per minute, but as fast as 15,000 rotations per minute(RPM'S). 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.


Each head in the hard drive is mounted onto a slider, which is mounted onto 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, which puts the head in the correct position. The reading head ( reading and writing heads are separate) floats about 2/1,000,000 of an inch above the disk surface. As it passes over the appropriate disk sectors, it interprets the magnetic pulses and converts them to electrical pulses that can be interpreted as 1s and 0s.

Although the head may look large, actually the sensitive part of the head is defined by micro lithographic methods so that the actual exposed portion of the head,which can either read or write the information on the disk,is very small-comparable to the dimensions of features on a microchip! As the manufacturing technology has improved over the years they have been able to shrink this area down to smaller and smaller sizes, and that is how the hard disk memories have risen to such large figures now. In addition they have increased the density of magnetic particles on the disk which makes the storage capacity larger. For example in 1991 the density of storage memory was about 0.l3 Gbits/square inch. In 1998 it was up to around 4 Gbits (30X the density!). At the same time the lithography limit of the sensitive head area was about 4.5 uMeters in 1991, and in 1998 was down to 0.5 uMeters, reduced by a factor of 9. So that is how the storage density of the disks is now up to 200 GB or higher..."



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