Windows will automatically allocate drive letters, and may re-allocate these when new drives are added (such as USB drives, etc.).
However you can allocate a permanent drive letter to a drive. I suggest that you choose a drive letter well up the alphabet, to ensure that it never gets in the way of other drives. I have allocated X: to my backup drive.
Make sure your external drive is connected and running.
Click Start, then right-click Computer.
In the menu, click Manage.
When the Computer Management window opens, click Disk Management.
Identify the external drive, and right-click on it. In the menu click on Change Drive Letter & Paths...
Allocate your chosen drive letter.