I had a similar issue, I fixed it by removing the first HDD and formated the second drive while it was alone in the NAS.
That worked great for me since I'm using "standard volumes", but wouldn't be of any help for those trying to make a RAID volume.
I've tried formatting one drive, rebooting and then formatting the other, it didn't work, then i tried to format both drives at once but that didn't work either.
P.S. my drives are a 1TB WD Green and a 2TB WD Green, I've even tried with 2x2TB WD green drives, had the same problem.
The firmware version is 2.00, maybe the update fixes that but i don't dare to install it as there's no way to go back to the old version and users reported a series of problems with the 2.02 version.