Scott Hanselman blogged a C# solution about this last week.
If you are handy with C# then its another good solution.
Source code is provided so if you have C# Express or Visual Studio then you should be good to go.
For everyone's reference:
I ended up settling on a 3 TB RAID because Mac OS X doesn't like any other solution. When using the above procedure and mounting the drive in a non default location, any item I would try to play (I was dealing with movies) would immediately disappear from iTunes. Poof! Gone. The item would still exist on the drive, just gone from the iTunes library.
So I thought - that sucks and went with a RAID.
I think Apple is going to have to deal with this at some point because it is rather impractical that one can't store their Music, Movies, TV Shows - anything that resides in a separate folder in iTunes on its own volume. My iTunes library is approaching 2TB at the moment and the only reason it hasn't passed the 3TB mark yet is because the realization that I will need an even bigger RAID (hateful!) to handle all of my content halted my quest to finally import and organize my 500+ DVD collection.
Oh, well... I guess I will continue to painfully browse the huge DVD storage folders where you can't search and even keeping things in alphabetical order is a chore. I had wanted to import all the movies and then properly tag them so I could search based on the score composer, director, actor etc. That would be sweet.
Anyway - just wanted to make sure anyone arriving at this page knows the solutions described are of no use in this particular case. Must be something about the way the iTunes database engine handles links. Apparently it is unable to follow them...
M
Best Answer
Well, you can manually do it by exporting the list of music in your library by right clicking the Music in the top left frame and selecting "Export." Then you could compare this to what is inside of your iTunes folder.
This seemed kind of tedious and figured someone out there had already come up with an easier method:
http://dougscripts.com/itunes/scripts/ss.php?sp=musicfolderfilesnotadded2
The above script looks promising. As usual, you should back up your files before doing either of the above just in case!
Good luck.