This is really a question of best practice.
In a datacentre environment whereby we have a single MD3200 DAS, and for the sake of the question, 6 expansion cages (MD1200s) how would you configure the RAIDS:
One single RAID per cage - i.e. a RAID10 consisting of all the physical disks in one cage, mapped as a single LUN to your servers/clusters. So essentially, we would end up with 7 RAID 10s. This way losing a cage will not affect your other RAIDs/cages.
Essentially the same, whereby we end up with 7 RAID10's but the logical disks are actually split across the cages. This way losing a cage may not result in the loss of any data, but could result in the loss of multiple RAIDs.