Drives that are connected and powered up, but not in use are called Hot Spares. They can be used immediately and automatically when the a drive fails in an array. The data on that drive is rewritten in the background.
A Warm Spare is connected but not powered up unless a failure occurs. When a failure happens, the Warm Spare is powered up and acts like a Hot Spare.
Machines Affected: PowerTower Pro with RAID