Scenario: You have a USB drive that your system does not like! It works in other systems but your system does not assign it a drive letter and does not let you format it or assign a drive letter in Disk Manager. It may not show up in RMPrepUSB either or may appear as 'Unmounted'. Your system may 'ding' when you insert it or remove it, but does not make it accessible.
First try backup up the USB drive (not an image copy, but a file backup), then delete the partitions on the drive (e.g. RMPrepUSB - Clean) and re-partition the drive. If you don't want to do this or it didn't work then (try this in order until it starts to work again)
1. Use Device Manager - View Devices by Connection. Expand the ACPI branch and keep going until you see the USB entries and expand those until you see your USB drive - click on Uninstall.
When it has finished. Unplug you USB drive and click on Action - Scan for Hardware Changes. Then plug the USB drive back in.
2. Run USBDeview in Administrator mode. Try the Disconnect, Uninstall and Disable options on the 'bad' USB drive. USBDeview may show it as having a drive letter even though the drive is not listed in Explorer!
3. If still a problem, download and run USBOblivion which removes all USB storage devices from the registry. After doing a trial run, tick the box to run it for real. make a note of where the .reg backup was stored. On my system this stopped all drives from working. REBOOT your system.
Afterwards, repeat Step 1 if necessary. Check that all your USB drives can be detected correctly now.
If the system is in a worse state than before, use the .reg backup saved by USBOblivion (double-click on it to re-install the registry entries)