I can't remember exactly how to do it but basically you plug in your external drive to your mac mini, and then in itunes you go to advanced settings and one of your choices for storing your music will be the hard drive. In other words your pointing your library to the hard drive, so when you load your music it will automatically be stored on your new drive. It works like a champ. Oh, and once your hard drive is recognized you can drag and drop "copy" all of the music that is already stored on your mac mini. Hope this helps.
Greg
Greg