Abstract: A disk controller which minimizes the rad operations required during parity writes of less than a single stripe. The disk array can be generalized as including N+1 disk drives with each stripe including a block on each of the N drives storing data and a block in 1 drive storing parity information. The present invention operates during writes to K blocks of a stripe where K is less than N, i.e., a partial stripe write. If K is greater than (N-1)/2, the N-K unwritten blocks are read in order to compute the new parity information before the actual write take place. If K is less than or equal to (N-1)/2, then the controller only reads the old parity information and the old data in the sectors to be written to compute the new parity. Thus the number of reads required to compute the new parity information is minimized.