The disk controller is circuitry on the computer's motherboard or on a plug-in circuit board that controls the operation of your hard disk drive, floppy disk drives, or both. When the computer wants to transfer data to or from the disk, it tells the disk controller. The controller in turn sends electronic commands to the disk drive making the disk spin and move its magnetic heads to the proper location on the disk. The controller then transfers the data between the computer and the disk drive. The computer's OPERATING SYSTEM and BIOS issue commands directly to the controller to BOOT the computer and to access files stored on the disks.
There are several different types of hard disk controllers, based on the electronic method used to encode data on the hard disk. The type of controller must match the type of hard disk you have-an MFM drive must be hooked up to an MFM controller, an RLL drive needs an RLL controller, and a SCSI drive only works with a SCSI controller. If you add another disk drive to your computer because you need more disk storage, you must know what kind of disk drive and controller you currently have, and the new disk must match your existing controller.