Spare disk units can reduce the time the system runs without mirrored protection for a mirrored pair after a disk unit failure. If a disk unit fails and a spare disk unit of similar capacity is available, that spare disk unit can be used to replace the failed disk unit. Using the DST or SST replace option, the user selects the failed disk unit to replace, then selects a spare disk unit to replace it. The system logically replaces the failed disk unit with the selected spare disk unit, then synchronizes the new disk unit with the remaining good disk unit of the mirrored pair. Mirrored protection for that pair is again active when synchronization is completed (typically, in less than an hour). However, it might take several hours from the time that a service representative is called until the failed disk unit is repaired and synchronized, and mirrored protection is again active for that pair.