A disk drive system includes a disk drive controller system having separate head positioning and data transfer subsystems and supporting up to four disk drives, two of which may be removable and two of which may connect through a standard ST506 interface. The system provides a read and write protected cylinder for removably drive and a read only cylinder for removable drives which may store a unique serial number as well as other write protected data. Each removable disk drive may includes a register storing an identification code which may be used to control operation of the drive.