When you are selecting a primary volume for a mirrored pair, a list shows all the eligible volumes.

Any volumes that are not eligible to be used do not display in that list. Volumes may not be eligible for any of the following reasons:

For DE6400 and DE6600 controllers, the primary and secondary volumes of an asynchronous mirrored pair must match the same protocol, tray level, segment size, security type, and RAID level. Non-eligible asynchronous mirrored pairs will not appear in the list of available volumes.