The hard drive controller "manages" or operates the hard drive. When the CPU needs to do a read/write operation, it talks to the hard drive controller instead of the hard drive itself. This saves the CPU time, and allows the hard drive controller to worry about telling the hard drive what to do.

The hard drive controller can be built into the hard drive, or it can be on an expansion card that takes up an expansion slot. The most common flavors include: IDE or integrated drive electronics and SCSI for small computers system interface. Both controllers are built into the hard drive.

The IDE controller connects with a ribbon cable to an IDE connector which is built in the motherboard. Each IDE connector can handle up to two IDE hard drives. If your motherboard does not have an IDE connector, you can buy an expansion card to plug your hard drive into. IDE provides for a data transfer rate of up to 10 megabytes per second. A newer version of IDE called EIDE for enhanced IDE, allows you to add more hard drives with larger capacities with a data transfer rate of up to 16 mebabytes per second.

The SCSI controller connects with a ribbon connector to an expansion card as motherboards do not provide a built-in SCSI connector. Each SCSI expansion card can run from 7 to 21 devices depending on the flavor. SCSI is faster than IDE and provides a data transfer rate of up to 40 megabytes per second, but is more expensive than IDE.

Older PCs use ST-506/412, and ESDI, hard drive controllers with MFM and RLL drives. These hard drive controllers are not as efficient as IDE or SCSI, and cannot be mixed with IDE or SCSI drives.