A hot spare disk is a disk or group of disk that is ready and waiting to automatically or manually, depending on the hot spare policy, replace a failing or failed disk in a RAID configuration.

Using the RAID algorithms, the missing data from the faulty drive is reconstructed and written to the hot spare. When the bad drive is replaced, it then becomes the hot spare. If a hot spare is not used, then the faulty drive must be manually removed and replaced with a new one.

The hot spare disk reduces the Mean Time to Repair (MTTR) a RAID redundancy group, thus reducing the probability of a second disk failure and the attendant data loss that would occur in any singly redundant RAID (e.g., RAID-1, RAID-5, RAID-10).

Since hot spares generally aren't used until a drive fails, it is a good idea to test them regularly to make sure they're fully functional.