I agree with Answer3. The driver that comes with NT4 out-of-the-box (with no Service Packs) does not support drives larger than 8GB. So, NT will only see 8GB of your 20GB drive.
You can install NT on the 8GB drive C:, then install at least Service Pack 4. It was on SP4 that the driver was changed to support larger drives. But, since SP6a is out, use that one.
Then go into Disk Administrator and create the other drives you want, and they can be larger than 8GB.
Now, there is a Technet article Q197667 on this you can read:
http://support.microsoft.com/search/preview.aspx?scid=kb;en-us;Q197667
(please remove any spaces)
hope this helps