Sorry, I should have been more specific. The firmware for the DNS-321. Some users report their 1.5TB drives working, others do not. I am not sure, but there are different hardware revisions of the DNS-321 which may or may not be contributing to the incompatibilities. My DNS-321 has hardware revision A2. I found this on the box near the serial number. Maybe in the future firmware update this will be addressed. As mentioned in other threads, its worth while loading the latest firmware and try formatting with different web browsers, Firefox or IE6/IE7 may provide different outcomes.