Windows often fails to auto-assign adrive letter to external drives, but you can do it manually:
Control Panel > Admin Tools > Computer Management > Disk Management
Find the ribbon in the bottom-right pane which represents that ext drive.
Right-click it and choose "Change Drive Letter & Paths.
Click "New" then choose a drive-letter from the drop-down box.
This is th solution i found on another thread posted... It worked..