How Do I Point iTunes to use my backup ext drive?

I use iTunes on a Mac Mini with 2 external 500GB disk drives. I use one of the external disk drives strictly as a backup. All I do to backup my iTunes information is I copy the complete contents of the iTUNES folder on my primary drive to the backup drive by the drag & drop (copy) feature. As you Mac users know, the iTunes folder contains 4 sub folders with the Music Library, Album artwork and XML file.

Well the dreaded day has come and I suffered a hard disk crash. No problem as my backup disk contains all of my needed iTunes files. However when I try to access my playlists, iTunes tells me it can't find the song title. I can change the location of the song title to the backup drive and it finds it successfully, but with over 4,000 songs ripped I can't imagine that I have to do this for every song. I changed the Advanced Preferences in iTunes to point to the backup location, but I think this is only to tell iTunes where to rip new music.

How do I tell iTunes to reference the backup drive so that it can find my playlists and music files?

Thanks in advance,
Brian ...