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That might work but iTunes finds files by storing the path to it in the library. The backup would have to be an exact duplicate having the same name and same file structure as the original which is doubtful. If you renamed it to match the old one that might work. If you go to the Music/iTunes folder you can open the "iTunes Library.xml" file and see what path it is looking for.
Interesting, I just did a test by renaming my music drive and iTunes followed that by renaming the path of the song. Evidently it is now using something other than the name to keep track of them. Not sure it is smart enough to do that if you switch to a different drive.
If you do an "add to library" then you will have two listings for each song, one of which iTunes can't find because it isn't there. You can get rid of the dead tracks by using this
http://dougscripts.com/itunes/scripts/ss.php?sp=removedeadsuper but might as well start over at that point. Hold down the option key as you start iTunes and it will give you the option to create a new library. Create one then just drag and drop the new drive onto the iTunes icon in the dock and it will add the songs. Only problem you will lose things like play lists, play counts, and ratings if that matters to you.
At least you have a backup. Looks like you need to get another drive ASAP so you have another. Too bad drive prices jumped lately.
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