If the drive contains a complete iTunes library, including the database files, launch iTunes with the Shift key held down, click on Choose Library, and point it there. If not, drag the music files to the internal drive and then into the open iTunes application window; if desired, you can create a new library on the external drive in the Shift-key selection screen.
... the iTunes files are on a seperate external drive. ..
did you follow ALL the steps as shown here...
Also... Have a look at these 2 Videos...