: This link also says 4: http://www.acnc.com/04_01_0_1.html 
 : I would reason that it is 3, 1 'real disk' and 1 disk as a raid 0. For example, 1 'real disk' of 240 GB is  mirrored on a raid 0 of 2 120GB disks. But apparenly they are counting that a RAID 0 must be implemented on   each one of the mirrors. I can't figure how can you call a 2 disk arrangement 0+1 as you either mirror or create one bigger disk but you can't do both.