As dynamox stated physical disk on array can be part of a raid group, you can create multiple luns against that raid group ID, and should check largest contiguous space available.
Attached below is the 'available storage' report from CLARiiON, the request is to create a 100 Gb LUN with RAID 5 id. But the capacity of each disk is 260 Gb. How does this work? Could some one explain this in detail?  [URL]
Are the disks shared with other Raid Groups?