This is why I'm really liking Crashplan as a storage service; they offer the ability to use their software to backup to your friends' drives (and vice versa). It perhaps doesn't work out well for everyone, but I have enough friends that keep up with the latest hardware that I am assured of finding someone with which I can trade backup storage. (My alternate plan was dropping off a disk / picking up the old disk whenever I visited my relatives every couple of weeks!)