I see two likely sources of your problem:
1 - The SAS drive(s) you are using is not compatible with the SAS controller (I do not believe you can use a 12G SAS drive in the XServe for example).
2 - More likely, you cannot mix and match SATA and SAS (the controller is set in one mode or the other at boot). I have used both SATA and SAS drives in my 2009 XServe without any problems... But all the drives (at any given time) used the same interface (SAS or SATA).
Hands down, the drives are the biggest issue I have had keeping my XServes running. Using NOS drives seems to work best - when I can find them.
Best of luck!