Same problem here, but I got it solved.
First, take out the NAS first drive with all your data (its safer and also the only way to make it work). Then put in the new one and power on your NAS. It will detect this disk and allow you to format it.
After formatting, shut down the NAS and put inside the other disk. It will become Volume_2 while the disk with your data will be Volume_2. It doesnt matters where you put it (L or R slot), it will be always Volume_2.
Now the will work as two separate units.
PD. If NAS have trouble detecting disk try to turn it on without disk an insert them after it completely boots.
Thanks for posting this, I was completely stumped with the same problem, just wanted to add a 2nd HDD to my existing 2TB HDD in the NAS which has worked fine since day 1, no install issues, just wanted a standard volume as the NAS is backup for a media server PC, so that should a drive fail in my media server I can replace it an restore from my NAS or vice versa, tried adding a 2nd 2TB HDD to my DNS-320 as a 2nd standard volume for extra storage, and ended up with the stuck trying tp format problem described, in my desperation I started reading posts here on the forums in hope of finding even a hint of how to resolve it...
I followed the steps you posted above, and voila worked a charm, simple, easy to follow, and took mere minutes.......... compared to the 2 days trying various things and scrutinising every page of the 2 different manual versions I've found for the DNS-320...
Thank You so much for posting your solution..
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Just to confirm, I took out the original HDD from slot 1, inserted the new HDD in slot 2 and re-started the NAS, waited til it ha fully booted (solid blue power LED, orange HDD2 LED), ran the setup wizard again, followed the steps through and this time it fully formatted the new HDD, once completed, powered down the NAS, opened it up again & re-installed the original HDD back into it's original slot, then re-started the NAS waite for it to complete booting (all LED's solid blue now
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checke on the PC and system see's both HDD's, an even still see's the original Volume 1 as Volume 1, and the new HDD as Volume 2...perfect.
Now why couldn't D-Link have put that in their manuals..
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?
Thanks again l3xx