The invention provides a method and system for reducing RAID parity computation following a RAID subsystem failure. Ranges of RAID stripes are assigned to bits in a bitmap that is stored on disk. When writes to the RAID are in progress, the bit associated with the range of stripes in the bitmap is set. When a failure occurs during the write process, the bitmap is analyzed on reboot to determine which ranges of stripes where in the process of being written, and the parity data for only those ranges of stripes is recomputed. Efficiency is increased by use of an in-memory write counter that tracks multiple writes to each stripe range. Using the write counter, the bitmap is written to disk only after each cycle of its associated bitmap bit being set to a value of 1 and then returning to zero. The invention may be installed, modified, and removed at will from a RAID array, and this may be accomplished while the system is in operation.