A drive in a RAID configuration that sits idle until one of the RAIDed drives fails. The hot spare then assumes the role of the dead drive. When the dead drive is replaced you can make it the hot spare. (The old hot spare doesn’t revert back because it’s got data on it now.) Hot spares are most often used in RAID 5 configurations, but can also be used in RAID 1 or RAID 10. When a drive fails the hot spare must be populated with information regenerated from the other drives. During this time performance is degraded.