I experienced a similar problem with an external RAID 0 from LaCie. I was mounting via firewire 800 to a MacBook Pro for editing in final cut.
When LaCie was questioned about the cause the problem they cited a chipset incompatibility issue. Whether or not this is a just a unnecessarily complex way of saying "We have no idea" I don't like to speculate.
In the end we found a work around. We plugged the drive into a borrowed MacPro (via firewire) and 30 minutes later it re-appeared on the desktop, looking sheepish.
I suggest trying to get a replacement drive. It's the only thing that worked for us.
Best of luck....