Drive Letter Missing from the List of Available Drive Letters
We have said it is very easy to add or change drive letter for certain partition in Disk Management, but sometimes you may find such a strange situation where a specific letter is not shown in the list of available drive letters and no other device has that letter assigned. The most probable reason may be that this letter is reserved for a removable device that was removed or is hidden.
Under this situation, if you want to assign the very letter for the specific drive, you may need to make some modifications in Registry. However, as a slight mistake in Registry may cause data loss or system crash, it is very necessary to backup the registry before doing any modification. For detailed steps, please see How to Backup and Restore the Registry in Windows.
Then, launch Regedit, navigate to HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices, and see if a device is shown as being mounted at the specific drive letter. Next, right click the drive letter that you want and choose "Rename" from the right-click menu to change the letter to any other unused letter, which will free up the selected one.
rename drive letter in registry 9
After reboot, you can add the released letter for your drive. And next, let’s see the fourth reason for losing drive letters.