Instructions
Unity enforces a hot spare policy of one out of every thirty drives of a given type.
Initial Release - This means if there are thirty drives available, twenty-nine can be used for a pool since the last one must be left unbound as a hot spare. If thirty-one drives are available, two must be left unbound as hot spares. In order to create a thirty-drive pool, thirty-two drives are required, as two hot spares are needed.
Unity OE 4.0.1.8194551 or later, the hot spare policy reserves one out of every thirty-one drives. This means
if there are thirty-one drives available, only one is must be left unbound for the hot spare and the remaining 30 can be used in a pool.
Additional Information
see the Unity High Availability paper in the Hot Spares section on Page 12:
https://support.emc.com/docu71319_Unity:-High-Availability---A-Detailed-Review.pdf?language=en_US