Hard Disk
Today, the hard disk is the primary storage device for all computers. Since hard disk stores so much data, it is some time called a mass storage device along with tape, optical disk and other media that can store a great deal of data.
A hard disk is a stack of metal platters that spin on one spindle, like a stack of rigid floppy disks. Each platter is coated with iron oxide and the entire unit is encased in a sealed chamber. Unlike floppy disks, where the disk and drive and separate the hard disk drive, is the whole unit. It includes the hard disk; the motor that spins the platters and a set of read / write heads.
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| Hard disk |
The rigidity of hard disks allows spinning much faster then floppy disk. A hard disk spins at around 3600 RPM instead of a floppy disk 300 RPM. Some hard disk spins as fast as 5400 RPM because the hard disk spins faster. The drive heads can use a lower intensity current to polarize a spot on disk. When the drive heads read data from the surface of the disk, the speed of the disk amplifies the intensity of the current induced in the read head.
The 80 tracks on a floppy disk are about 74/10,000 inch apart (135 track per inch). Hard disk can contain over 1000 tracks per inch. All other factors come together to allow hard disk to store much more data per square inch than floppy disks.
The more surface area provided by multiple platters are the main reason that hard disks can store so much more data than floppy disks.
Hard disk consists of more than one platters stacked one a top another, with disk 0, 1,2,3,4 and so on. Some disk drive hold 12 disks, but both sides of their disks are not always in used with hard disk. The number of sides that the disk uses is specified by number of heads. For example, a particular hard disk drive might have sin disk platters (i.e 12 sides), but only ten heads, indicating that two sides are not used to store data. These are the top sides of the top disk and bottom side of the bottom disk.
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| Hard Disk |
Because hard disks are actually a stack of platters, we used a new term cylinder to refer to the same track across all the disk sides. O track is the most tracks on every disk in cylinder.
Like floppy disk, hard disk generally store 512 bytes of data in a sector, but hard disk can have more sector per track i.e. 54, 63 or even more sectors per track.
The computation of a hard disks capacity is identical to that of floppy disks.
Example:
Capacity of 541 MB hard disk can be computer.
Cylinder = 1632
Head sides = 12
Sector per track = 54
1632 cylinders x 12 heads = 19,584 tracks
19584 tracks x 54 = 1057,536 sectors
1057,536 x 12 = 541,458,432 bytes
= 541 MB
