Provided is a RAID system that includes a memory device including a plurality of nonvolatile memories storing data chunks and first and second parity bits corresponding to the data chunks, a RAID controller configured to control a RAID operation and a rebuild operation of the memory device, and a host configured to communicate with the RAID controller at an input/output (I/O) rate. The RAID controller is configured to perform a first rebuild operation using the first parity bit and a first policy having a first rebuild rate, to perform a second rebuild operation using the first and second parity bits and a second policy having a second rebuild rate, to apply a first sub-rate as the first rebuild rate until the I/O rate reaches a predetermined value, and to apply a second sub-rate as the first rebuild rate when the I/O rate is greater than the predetermined value.