A secondary storage device for a digital computer includes a housing, an attachment means coupled to an outside surface of the housing for allowing said housing to be easily attached to the computer system, a primary disk drive disposed within said housing, at least one back-up disk drive disposed within the housing, mirror circuitry disposed within the housing, and coupled to the primary disk drive, and the at least one back-up disk drive, bus connection means coupled to the mirror circuitry for connecting said secondary storage device to a main bus of the digital computer, and timing means coupled between the mirror circuitry and one of the back-up disk drives for creating mirror back-up images of the primary disk drive at pre-determined intervals.