A hot spare disk should always be available on your storage system in the event that an active disk fails.
Hot spare disks are disks held in reserve globally, in case an active disk fails.
Hot spare disks in a storage system do not belong to any particular volume. In fact, any disk in the storage system that has not yet been assigned to a volume (and has not been marked bad) is treated as a hot spare.
If the storage system has multiple volumes, any available spare can replace a failed disk on any volume, as long as the following conditions are true:
Always keep at least one hot spare disk in the storage system. This ensures that a spare disk is available at all times. As soon as an active disk fails, the storage system automatically reconstructs the failed disk by using the hot spare. You don’t have to intervene manually—except to replace the failed disk after the reconstruction is complete.