Can't assign a particular drive letter to a partition

I want to assign a particular drive letter (R) to a partition on a drive for compatibility with another computer system. However when I attempt to change the drive letter using Windows Disk Management, "R" is not shown as a choice. There is no other disk shown in the listing of Volumes with the drive letter "R", and every other unassigned letter is available, so why isn't "R" shown? If indeed R is assigned to something, how can I find out what it is?

Doing a drive cleanup will free all the drive letters associated with non-present devices and thereafter you should be able to assign whichever free drive letter you want to assign to the partition. With this small utility doing your bidding , there is no need to do any registry editing.

Download v 1.1.0 of the drivecleanup.zip from Drive Tools for Windows.

Unzip it to a folder, say drivecleanup.

You will have two folders Win32 and x64 each containing DriveCleanup.exe for 32 bit and 64 bit Windows respectively.

Remove all USB storage devices from your system (except your Keyboard and mouse) and reboot.

Right click on the DriveCleanup.exe and run as administrator. (Use the *.exe file appropriate for your bit version of Windows.).

After the clean up act, reboot. Now you should be able to assign R or any other free letter to your drive/partition.

( When you plug in your USB devices after this cleanup, these will be installed afresh. What you have done is to remove all non-present devices and drive letters associated with them and start with a clean slate. Hopefully that should resolve the problem.)

Brink -
Thanks very much for your suggestion. I checked the registry and it lists the devices and letters that have been assigned, but *nothing* is listed for R. However I then tried OPTION THREE (command prompt method) and manually assigned R to the volume in question, which worked! I don't know why Windows wouldn't list R as available, but in any case the problem is solved.

jumanji-
Thanks also for your suggestion, but as you can see the problem is now resolved.

Again, thanks to you both. I appreciate the help.