A method for automatically diagnosing faults a data storage system. The system includes a plurality of enclosures each having: a primary port; an expansion port; a plurality of disk drives; and a link control card coupled to the primary port and to the expansion port and the plurality of disk drives. The link control card includes a cut through switch having: disk drive port error counters for counting at ports of the plurality of disk drives; a primary port error counter for counting cumulative errors at the primary port, and an expansion port error counter for counting cumulative errors at the expansion port. The primary ports and expansion ports are serially interconnected to the storage processor through a fiber channel loop. The method sequentially reads counters in each one of the enclosures to determine whether errors counted in any one of such counters exceeds a predetermined threshold over a predetermined period of time.