I used to try to do that, but I finally gave up and bought a hard drive big enough to hold everything at once. I'm really glad I did. Right now, I've got about 15K tracks, and they use up about two-thirds of the space on a 120 GB drive. In a couple of years when my collection gets too big, I'm sure hard drives will be cheap enough that I won't mind swapping to a bigger one.
How big is your collection?
If you really have to do what you suggest, the only way I can see to do it would be to create symbolic links for certain artists within the iTunes Music folder, and have those point to another disk.
For example, assume you have tunes by two artists, ArtistA and ArtistB. You want all the tunes for ArtistA to live in your home directory, and all the tunes for ArtistB to live on some other disk. Assuming you have room, let iTunes put all the stuff on your home directory. Then, create an iTunes directory on the other disk and move the ArtistB directory there. Use Terminal to create a symbolic link in your home disk iTunes Music folder that points to the other folder. I haven't tried this, but I think iTunes will think that the ArtistB directory is really on your home disk, even though it's really on the other disk. If you add new tunes to ArtistB, then I think iTunes will put them on the other disk. Of course, if you add tunes by ArtistC, they'll go on your home disk.
If you try this, please let me know if it works or not.
If you haven't used symbolic links before, I advise you to not try this. That's one reason why I didn't give a command line syntax for doing this.
Pat
