If your new USB disk is not recognized by a drive letter on your Windows computer, or if it was recognized before but not anymore, you need to re-assign a drive letter to the disk.
1. Check your computer's drive status - Double-click on the [My Computer] icon.
2. Note that there should be an "E:" drive listed, but it is missing - close the window.
3. Right-click on the [My Computer] icon, select [Manage]
4. From the Manage menu, select [Disk Management] - a panel displaying the physical drives and partitions will appear.
Note that there is a drive listed with a name "WD_Backup" but it has no drive letter. This is the one to which you need to assign a drive letter.
6. Right-Click on the drive panel with no drive letter, select [Change Drive Letter and Paths].
Click [Add], then select a drive letter from the list of available letters. Don't ever use "A", "B", or "C" - use whatever next highest drive letter is available. Click [OK] to complete.
Return to the disk management panel after the drive letter has been added. Double check that the drive letter is assigned.
Close out the Disk Management panel. Double-Click [My Computer] and view the drive letter has now re-appeared as normal.
Tips
Check all your existing drive letters before starting, note the warning below.
If you want the USB drive to be a certain letter, say "D", but "D" is already taken, you can use these same steps to re-assign a drive letter, thereby freeing up "D" to be assigned to your USB disk.
Warnings
Don't ever try to use drive letters "A", "B", "C", or any existing drive letters, as it will cause problems.