Each file has an "owner" ... no matter what the file type is. Since QT is a valid mp3 player as well, mp3's can have an "owner" of QT and will show up with a QT logo instead of iTunes.
To change the files on your external to be owned by iTunes, select an mp3 file (or all the mp3 files), right-click (or control-click, if you have a one button mouse) and select "Get Info". About mid-way down the info dialog you'll see something that says "Opens With". Right now, it would be set to QT ... you can change that to iTunes. Beneath that will be a "Change all" button that will associate all mp3 files with iTunes.