A mass data storage system including a hard disk drive comprising heads and platter surfaces determines when a head of the disk is faulty and the disk continues to operate as a partially failed disk with respect to the remaining heads which are not faulty. A striped parity disk array comprises disks capable of operating as partially failed disks allows copying of data from the platter surfaces not associated with a faulty head of a partially failed disk to a spare disk which reduces the amount of data that must be rebuilt in the rebuild process, thereby reducing the amount of time the array spends in degraded mode exposed to a total loss of data caused by a subsequent disk failure.