Yup, as noted you can do this.
I have all of my music on a portable external firewire disk. When I need to sync my iPod, I just daisy chain it onto the external firewire disk and everything works flawlessly.
I just wish notebook hard drives would come up to speed (figuratively and literally). They are slow and small in comparison to desktops and, IMO, are one of the principal bottlenecks in speed for many day-to-day activities on a portable.