A two-dimensional parity method and system that provides four-disk fault tolerance in an array of disks, such as a RAID system, is presented. The method includes arranging strips containing data in the disk array into horizontal, diagonal, and stepped parity sets, each parity set including at least one data strip as a member and no single data strip is repeated in any one parity set. The horizontal, diagonal, and stepped parities are than stored in respective disks and stripes of the array. In the event of up to a four simultaneous disk failures, lost data can be reconstructed by using the corresponding stored horizontal, diagonal, and stepped parity information and the data stored on the remaining intact disks.