in this article: http://blogs.technet.com/b/yungchou/archive/2012/08/31/windows-server-2012-storage-virtualization-explained.aspx it says: Mirror - This is a mirror set without striping or parity by duplicating data on two or three disks, similar to RAID 1. It increases reliability with reduced capacity. This configuration requires at least two disks to protect data from a single disk failure, or