1. In combination with a storage subsystem having a control unit and a plurality of disk storage disk storage drives connected to said control unit wherein said control unit includes control means for controlling the transfer of data in the form of segmented system records between a CPU of a data processing system and said storage subsystem through said control means in response to commands supplied to the control unit specifying CPU addresses of said system record, the length of said records, and the physical storage location in said disk drives where said segments are to be stored, the improvement comprising: A. a generator for (1) generating a check sum segment corresponding to the logical combination of all segments of a record simultaneously during transfer of a system record from said system to said drives, and (2) recreating an unavailable record segment from other available records segments and said check sum segment simultaneously during a transfer of said available segments and said check sum segment from said drives to said system; and B. circuitry under control of said control means including register means addressable by said control means for transferring (a) signals to and from said genertor corresponding to segmented system records being transfered, (b) a check sum signal to a selected drive; and (c) a recreated unavailable system record segment to said CPU.