A redundant array of independent disks (RAID) storage device including; a memory device including first memory devices configured to store at least one of data chunks and corresponding parity (data chunks/parity) and a second memory device configured to serve as a spare memory region, and a RAID controller including a RAID internal memory configured to store a count table and configured to control performing of a rebuild operation in response to a command received from a host, wherein upon identification of a failed first memory device, the RAID controller accesses used regions of non-failed first memory devices based on the count table and rebuilds data of the failed first memory device using the second memory device.