How can I get iTunes to use a library on an external HD? I've configured iTunes to use a library on an external drive by going to Preferences > Advanced and setting the "iTunes media folder location". It was working fine for months.

Then last night I didn't have the external HD with me and I needed to do something with iTunes so I temporarily pointed it to the default media folder location.

However, now I can't get iTunes to go back to the external HD.

I set it in the preferences, but when I launch iTunes none of the songs, movies, or playlists from the external drive are present.

So iTunes is configured to use the media folder on the external HD, and looking at that filesystem I see that my stuff is in there, but I can't get to it.

I tried restarting the machine.

Mac OS 10.7.5.

iTunes 10.
Hold down option when clicking on the iTunes icon and browse to the media library on the external.  It will set this as the new default, which is what you did when creating a new library "temporarily".