Windows will sometimes fail to assign a drive-letter to an external drive, even though that same drive may have worked okay before on the same computer.
Go into the Disk Management console:
"Control Panel > Admin Tools > Computer Management > Disk Management".
In the bottom-right pane, find the ribbon which relates to that drive.
Right-click on it and choose "Change Drive Letter and Paths".
Click "New", then choose a drive-letter from the drop-down alphabet.