At our facility I used a Retrospect LTO4 system to automatically backup all my RAIDs - whether it was Xserve RAIDs or internal RAIDs in the Mac Pros. It was very helpful when we had the inevitable drive failure. BTW, I did see a two drive failure on RAID 5 systems a couple of times. Naturally, one two drive failure was on the Autodesk Smoke system that was not compatible with the Retrospect backup. Figures.