I remember reading a post a while ago (which I can't find anymore) about someone complaining you can't have multiple music directories with slimserver/squeezebox.
But, you can, and it is actually easy.
First, go select the other directory (the one that is not pointed to in slimserver), and right click it, and select create shortcut. This will make a shortcut to that directory.
Now, copy the shortcut into your music directory, rescan and the directory will now show up.
You could also use this over multiple hard drives.
If this is common knowledge, then you can ignore, but I just figured it out today.
Randy