I've had the N5200 for a few years, and it worked flawlessly with 5 500GB WD Caviars (WD5000AAKS).
Then I upgraded to firmware 3.00.04.5, and within a few months one of the drives was showing bad blocks and went bad.
I replaced all 5 with 5 2TB WD Caviar Black drives (WD2001FASS).
Well, after a several months, i noticed the RAID was degraded (RAID 6). TWO of the drives were showing problems. I couldn't check them through the the Thecus UI, so I took them out and tested them on a PC, and they show over a thousand bad blocks (probably more, but I stopped the test).
I've replaced the two failed drives with identical 2TB Caviar Black drives and now I am rebuilding the RAID.
Is this just bad luck and coincidence, or is this firmware doing something bad to the drives?