Been there, done that... on both drives I have successfully performed WD diagnostics, successfully formatted both drives on a separate PC, successfully performed full surface scans, and even successfully ran SpinRite on both drives. The drives are perfect.
The DNS-321 is junk, with garbage firmware, and is poorly supported and documented, and D-Link KNOWS IT judging by the tons of complaints regarding this dead weight all over the Internet. No more D-Link hardware for me and I am now a committed vocal advocate to my colleagues/friends to stay away from D-Link, especially after reading the sarcastic/rude responses from D-Link employees on these forums. I've also escalated their sarcasm higher up in the company and will see what effect that has, if any. J-U-N-K.