This counter reports the total number of hours the physical drive has been spinning since it was stamped, either at the factory or when diagnostics were first run on it. An admin uses this reference time as the denominator when calculating error rates for other physical drive event counters, such as read or write errors per operating hour. For example, dividing cpqFcaPhyDrvHardReadErrs by this reference hours value lets the admin compute an error rate to compare across drives of different ages.