The administrator should rebuild the volume to restore it after replacing the failed hard drive. A volume is a logical unit of storage that can span across multiple physical disks. A volume can be configured with different levels of RAID (Redundant Array of Independent Disks) to provide fault tolerance and performance enhancement. When a hard drive in a RAID volume fails, the data on that drive can be reconstructed from the remaining drives using parity or mirroring techniques. However, this process requires a new hard drive to replace the failed one and a rebuild operation to copy the data from the existing drives to the new one. Rebuilding a volume can take a long time depending on the size and speed of the drives and the RAID level.