Abstract: A method and apparatus by which the partial error check value for all preceding data in a sector is updated each time a new data item is written into a data buffer during a transfer from a host computer to a system storage device. The partial check value is written into the buffer location following the location containing the most recently buffered data item from the associated sector. Thus, after the entire sector has been transferred into the buffer, the final check value for that sector will be available in the buffer location following the location containing the last written data from the associated sector. Upon readback of a sector from the storage device, the sector size is longer by one data item since the appended check value is treated as the last data item of the sector. When the sector is read back from the storage device, the check value is again updated with the entry of each data item.