Hard Disk

It is a non - volatile storage device which stores digitally encoded data on rapidly rotating platters with magnetic surfaces.The usage of hard disk began in 1956 with the shipment of an IBM 305 RAM AC computers.See This : http://www.youtube.com/watch?v=9eMWG3fwiEU.
There are four main types of hard disk:
1. IDE - Integrated Drive Electronics.
2. SCSI - Small Computer System Interface.
3. SATA - Serial Advanced Technology Attachment.
4. PATA - Parallel Advanced Technology Attachment.