I'm not sure what you're asking for here... If you have music files already on your internal drive and you want to move them over to the external drive and only have MC recognize them in the one location, on your external drive, the easiest way would be to delete all your library entries then go to Tools -> Import. Choose to either Import a single folder or Auto-Import and click Next.
For a single folder browse to the folder you want to import the pick the appropriate file types and click finish, after taking note of the other options available - "Ignore files..." and "Ignore special"...
For Auto-Import click add and browse to each location you want MC to always update the library with when files are added. Take note of the options tasks at the bottom and click Finish.
ALWAYS TAKE A BACKUP OF YOUR LIBRARY before making drastic changes like this so you can back pedal if necessary. And note this method will only work if you have tagged audio files MC can use to rebuild the database in the library with. (Tagged = MP3s, for example, which contain all artist, album, song etc. information embedded in the files.