My wife has a MacBook Air, which she loves dearly, but sadly we didn't anticipate just how much space she needed when we got the 128GB version.

However, she just started recently using Rdio, and so she doesn't really have much need for keeping all of those music files on her hard drive. Moving them to an external hard drive would be fine for those times that she needed them.

That's easier said than done though. It's easy enough to move the entire library to an external drive. But she doesn't want that, because she uses iTunes all the time for books and doesn't want to hook up the external drive each time she does that.

Here's the best method I could put together for moving the physical MP3 files to the external drive, update the iTunes library to point to that location, and leave everything else as it is.

  1. To be safe, backup both the iTunes Library.itl file, and the iTunes Music Library.xml file. Copy the iTunes Music Library.xml file and name it something new.
  2. In iTunes, make sure the organize library and copy files options are disabled (under Preferences->Advanced).
  3. Copy all of the MP3s to the external drive. Any directory will do.
  4. Delete all of the music from iTunes, and allow it to move those files to the trash (since we just copied all of that to our external drive). Quit iTunes.
  5. Open up the new iTunes Music Library.xml file that you copied in step one. You'll see repeated references to "file://localhost/", which continues with the path to your local music directory. Replace that URL with the new directory, which should look something like "file://localhost/Volumes/External%20HD/Music"
  6. Open iTunes again and click File->Library->Import Playlist. Select the .xml file we just modified.

And that's it. The only issue I see with this is that I believe if you enable the option to organize the library, it will copy all of the files back to the local hard drive.