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Since you've mentioned MS SQL - take a look at the Reference Architecture and Best Practices from Powerlink.
You can find White Papers at this location on Powerlink:
Home > Support > Technical Documentation and Advisories > White Papers > All White Papers
Also - as you can see in the Reference Architecture you could actually configure your system with just the S-ATA hot spare since it can stand in for a failed SAS drive as well.
Of course in case of a failure your performance would drop the S-ATA level, but you would have more capacity