The hard disk

The hard disk

 The hard disk's role is to store data permanently. It uses a set of disks called platters which store data on tracks. The data is written on all platters (usually 3) on cylinders (3 tracks, one on each platter) at the same time. This greatly improves read/write times. The operation is accomplished by using 3 writing heads and 3 reading heads that constantly move on a ray of the disks to read/write the data. All disk surface is covered by these heads because of their spinning. The hard disk access speed and read/write performance is pretty much related to their speed. Typically a fast hard disk uses a motor that spins the platters at more than 7200 rotations per minute.

 

Also there are many technologies available like ATA100, ATA133 and the SATA which are the fastest available.
Usually, the problems that a hard disk could have are strongly related to the way it was used. Hard disk are pretty sensible, so try not to drop them down when installing. If you do this, you could end up with bad clusters (physical damaged areas of the disks) that will shorted it's life.
When buying a hard disk, consider it's capacity (number of GB that can be written on it), it's buffer size (a temporary memory that greatly improves read/write speed), it's speed (number of rotations per minute - faster ones like 10000 rotations per minute could need a cooling system) and it's interface (ATA/SATA - to match your motherboard interface).