Isn't it something to do with the fact that the CPU is involved to a greater extent with USB 2.0 whereas FW400 isn't as CPU-intensive? That and the more advanced up and down stream of the FW400 tech. Anyway, yeah - a FW400 external hard drive was always more reliably quick for me. DVD burners as well. There's almost no comparison between FW400 and USB 2.0 on my old LaCie burner. Things have improved a lot since then, though, because I've got a USB 2.0 burner now that operates perfectly well.

P.S. Anyone else remember how unreliable burning CDs over USB 1.1 was? Or was that just me? Made a lot of coasters back then...