Assigning a hot spare gives extra protection against data loss. In normal conditions, a hot spare disk is unused and does not store any data. When a disk in the RAID group fails, the hot spare disk automatically replaces the faulty disk. QTS copies the data to the spare disk in a process called RAID rebuilding.
The spare disks are added to the RAID group. The disk appears as a green Spare in the disks summary at Disks/VJBOD.