A computer data storage device made up of both solid state storage and rotating magnetic disk storage maintains a fast response time approaching that of a solid state device for many workloads and improves on the response time of a normal magnetic disk for practically all workloads. The high performance is accomplished by a special hardware configuration coupled with unique procedures and algorithms for placing and maintaining data in the most appropriate media based on actual and projected activity. The system management features a completely searchless method (no table searches) for determining the location of data within and between the two devices. Sufficient solid state memory capacity is incorporated to permit retention of useful, active data, as well as to permit prefetching of data into the solid state storage when the probabilities favor such action. Movement of updated data from the solid state storage to the magnetic disk and of prefetched data from the magnetic disk to the solid state storage is done on a timely, but unobtrusive, basis as background tasks of the described device. A direct, private channel between the solid state storage and the magnetic disk prevents the conversations between these two media from conflicting with the transmission of data between the host computer and the described device. A set of microprocessors manages and oversees the data transmission and storage. Data integrity is maintained through a power interruption via a battery assisted, automatic and intelligent shutdown procedure.