Well if they're not currently used, you won't "run out", your windows just thinks it is. We need to reset your drive letters. Let's start by trying this.
now, surf in the left pane up this tree:
Right click on that lil folder, export it to the desktop. This is our backup safetey, if windows reacts badly we can restore the value, and no damage is done, although there shouldn't be any problems here.
Now, delete this lil folder. So that the MountedDevices folder is gone. Then, reboot.
This, in effect, resets window's knowledge of drive lettering, forces it to detect and re-assign new drive letters. Your main drives should stay the same, if you didn't set them to special letters in the first place. Now, put in that USB drive, and see what letter it's given. It should, in theory, return to that F: like you want it. Put it in, take it out a few times, see if it stays at that F:, or if it increments again.
Report back what happens once you're ready.