Most personal computers have at least one hard disk drive. A hard disk drive provides a large quantity of non-volatile memory. This means that data stored in a hard disk drive does not disappear when a computer is turned off. A hard disk drive is used to store all kinds of files, everything from the operating system, application programs to user data are stored on a hard disk drive.
Compact sub-notebook computers now start to use solid state disks (SSDs) instead of hard disk drives. SSDs are more compact and requires less power to operate. However, SSDs have the typical problems of a flash memory device. For example, a section on an SSD has a limited number of write cycles.
More advanced system may have a RAID (redundant array of inexpensive drives) configuration. A RAID configuration combines multiple hard disk drives to improve the capacity, reliability and performance of the mass storage system. Many modern motherboards have RAID capabilities with some help from the operating system.
The spindle speed of a hard disk determines how fast data can be written or read from the platters. A consumer grade hard disk typically has a spindle speed of 7200rpm (revolutions per minute). Small notebook computer hard disks have a lower speed of 4200rpm to 5400rpm to save power. Hard disks for server applications have a high spindle speed of 10,000rpm.
Another factor that affects the performance of a hard disk is the seek time. This is the average amount of time to move the disk head from one track to another track. Most hard disk drives have an average seek time of 5 to 7 milliseconds. A millisecond is thousandth of a second.
Last, but not least, the interface that connects to a hard disk drive can also influence the overall performance. A common standard for personal computers is Serial ATA (advanced technology attachment) at a maximum transfer rate of 3GB/s (3 billion bytes per second).