You could keep all your data on one drive and everything else on the other. That way when you do back things up you just image the drive with the data and you're done. But more important is the fact that all the data is in one place! For me, I use a laptop (MacBook Pro), and my iTunes library is more than half my hard disk. So I've been contemplating using an external drive to house my iTunes library. Another advantage is that it would make the iTunes library more portable.