I bought 2 ministacks for my mac minis and they sucked. Every time I transfered a large file they would freeze up. I tried everything. Replaced the drives, tried the usb vs the firewire, tried on different macs same thing. They sat around for over a month before I tried calling OWC about the problem. I tried to return them. Then after a long conversation about returning them I was given to a tech who might be able to help me. He sent me some java app for updating the firmware or something and bam the problem was fixed. The java app was crap and didn't work properly so I had to call back to have him walk me throught the HACKS to use it but it worked.
If you buy the ministack make sure the firmware is updated or they include it. I am kind of pissed at them for not notifying their customers about this issue. Went through alot of hell with those things.