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Should one of the drives failed, parity data can be used to rebuild the failed data. Under RAID 5, parity data is stored across all drives in the array. This maximizes the amount of storage capacity available from all drives in the array while still providing data redundancy. RAID 5 requires at least three drives. The ACS-77100 allows user to set RAID 5 with three drives and the fourth drive as a hot-spare drive ready to be used for rebuilding data in case one of the other drives fails. The diag