I came across a rather annoying limitation of the Mac Pro RAID card last night that is worth being aware of if you have one of these in your big iron tower.
The max drive size is limited to 2.2 TB. I was unaware of this and pulled 4 x 1TB WD BLACK Caviar drives in RAID 0+1 and replaced with 4 x new WD RED 3TB Drives in RAID 0+1 (as I was running out of space). The drives maximum useable space is limited to 2.2 TB per drive; which is a limitation of the Mac Pro RAID Card.
There is no current workaround that I can find other than replacing the RAID card with an alternative model - which may or may not be supported by Apple (likely not).
There is a firmware update for the MAC Pro Raid card on Apples website - however, it does not address this limitation.
Just something to be aware of if you are putting new drives into a Mac Pro with the Apple Raid card. You might as well save a few bucks and either just use 2TB drives, or consider replacing the RAID card with something after market.