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About Discs © R. Craig Collins, 2008

A hard disk is a metal or glass disk coated with a magnetic material. As an area of the disk could have a positive or a negative charge, this could be interpreted as a 1 or a 0, suitable for computers to record data on.
hard disc

Below, a diagram of a disc showing the invisible tracks and sectors created during formatting. (Actually there are many more tracks and sectors than shown).
Tracks are the circular lines, similar to a bulls eye. The tracks are divided into sectors, which act as 'buckets' that can hold information. Sectors typically are too small to hold an entire file. (See note on bad sectors, below)

Tracks and Sectors

On older computers, and floppy disks, the locations of the various parts of a file are recorded in a File Allocation Table... similar to an address book.

Below, a disc storing two files; the dark file is fragmented, the light file is contiguous, or unfragmented. Fragmented files take longer to locate and load into memory, so many DOS and Windows disks need a defragmentation utility to optimize disk access.

FAT and fragmented files

Side view showing height of read write head, and various dangers
Disk read write head diagram
Sectors from time to time loose the ability to reliably store data. This is called a bad sector; bad sectors on a disc volume may be repaired with the Windows Error Checking tool.