High availability disk arrays

A method for handling data in a system having a processor for controlling access to a plurality of data storage disks, wherein storage sectors of a selected number of said disks contain user data and storage sectors of a selected one of said disks contain redundant parity entries which match with the user data entries in the sectors corresponding thereto, corresponding user data sectors and parity sectors in said disks forming identifiable sector groups, said method being used when said plurality of disks are being operated in a degraded mode wherein one of said user data disks is inoperative and the user data therein is reconstructed from the user data in the remaining user data disks and the parity entries in the parity disk, said method including the steps wherein

• before writing new data into a selected sector of an operative disk, writing the reconstructed user data for the corresponding sector of the inoperative disk into the corresponding parity sector of the parity disk to replace the parity entry therein;
• entering at a specified identification region of said parity disk, information identifying the inoperative disk whose reconstructed data has been written into the parity disk;
• writing the new data into the selected sector of the operative disk.