Synchronous mirroring - Why don’t I see all my volumes?
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 might not be eligible for any of the following reasons:
The volume is a non-standard volume, such as a snapshot volume or thin volume.
The volume is not optimal.
The volume is already participating in a mirroring relationship.