For some reason, Windows likes to reserve drive letters it previously saw.
This information is stored in the registry at
Simply delete the registry key of the unused drive letter at
\DosDevices\D: , then restart, and you'll be able to re-use the desired drive letter.
After doing this a second time, I found that it's easier if you do it this way:
Transfer all the files from the old data drive
D:\, to the new data drive
Do not reboot!
Make sure absolutely no programs are running that are accessing files on the old data drive. Shut down as many apps as possible. Then change the drive letter of the old data drive to something random
Now you can change the drive letter of the new data drive to the letter that was previously occupied by the old data drive,
Basically, change the drive letters with both old and new data drives connected; don't reboot until you have performed the drive letter switcheroo. That would avoid the problem I had above.