The hard drive stores all the computer's data and retains the data when the computer is turned off. A fast hard drive is needed to supply the CPU with data as fast as it needs it. Hard drive sizes are typically measured in GigaBytes. The larger the number, the more applications and games you can have installed. A hard drive interface divider into PATA(IDE),SCSI,SATA,and SATA2.
The SATA2 is the latest trend today.
measurement method for a hard disk:*Data rate - The data rate is the number of bytes per second that the drive can deliver to the CPU. Rates between 3 and 40 megabytes per second are common.
*Seek time - The seek time is the amount of time between when the CPU requests a file and when the first byte of the file is sent to the CPU. Times between 10 and 20 milliseconds are common.