I've got a drive (well, a separate partition actually) that has my music on it, and in theory I should just be able to hit play in Rhythmbox and it'll work. But for some reason, I need to personally go into the drive and just open the drive before rhythmbox will actually realise the files are still there.
A partial solution seemed to have been to put the following command into the startup applications list:
--mount /dev/sdb4
But that only worked when I logged in and out to test it. It doesn't actually work when booting up.
EDIT: I've just noticed that the drive is no longer being listed by Ubuntu automatically. I can see it if I look for it under drives, but its not automatically coming up.