Hardware faults in data storage systems are diagnosed. User I/O errors are received. Disk drive port error counters, primary port error counters, and expansion port error counters are read. A user I/O error threshold is modified based on the error counter readings. Depending on the type of errors counted, the user I/O error threshold may be increased or decreased. Once a first quantity of user I/O errors exceeds the modified user I/O error threshold, a faulty component is identified.