When you connect a new disk drive to your PC, Windows automatically assigns the next available drive letter. C is already taken, of course, by the system drive. So your external backup drive or USB thumb drive could end up as D, E, or F, depending on how many drive letters are already in use.
But what if you’d prefer a different drive letter? You might want to assign M to the drive that contains your digital music files, or H for your File History drive, or X for your top-secret X-Files.
No problem. Start by ensuring that the drive you want to work with isn’t in use and that no files from that drive are open. Next, open the Disk Management console (Diskmgmt.msc) and right-click the volume that has the drive letter you want to change. Click Change Drive Letter And Paths and then click the Change button, where you can choose from a list of available drive letters. (Don’t use A or B, which have historically been reserved for floppy drives and can confuse older software.)
You might have to restart to make the change, but once it’s complete, your new drive letter should be assigned permanently to your chosen device.