Hot Spare

A backup device such as a hard disk or controller that is in a standby mode, meaning it is online and available should the primary device go offline or fail. The system would detect the failure and automatically switch to the hot spare to use as a replacement. Hot spare is commonly used in RAID arrays where additional drives are attached to the controller and left in standby mode until a failure occurs.

Compare with hot standby.
See hot swapping, also called hot plugging.