Server 2003 Q? perhaps dumb, perhaps not.

Do this when no one will have access, of course.

1. Rename D drive to whatever and rename 300GB drive to new D drive.
2. Run ViseVersa to replicate directory (complete with permissions) to the new D drive.

Download ViseVersa here: http://www.tgrmn.com/index.htm?camp=goog_tplus
You can download a fully functional 30 day free demo.

Another option is to just replicate the files and folders and then point profiles to the new drive without changing the drive letter.