I went with WD HDD's because the problems with the Barracuda 1.5TB drives put me off buying any more.
I can't honestly remember when I bought those drives, but I think I had them a while before the problem started.
How about this as a test:
Unplug all 4 HDD's making a note of which one goes to which sata port, then plugging in one of my old Seagate 200's which is now lying around as a spare and installing an OS on it with both sticks of ram at standard settings and seeing what happens? Do you think this would be any use?
Thanks for your continued help with my issue, much appreciated.