The FireWire interface is supposed to be smart enough to figure this all out by itself. In fact I heard somewhere that if you copy from a FW HD to a FW DVD it can even bypass the computer so they talk directly to each other? The only thing I'd caution you about, is to ensure your external HD has its own power supply -- some little ones take all their power from the FW bus -- if you have to run it that way, make sure the HD is first on the chain. FYI I go iBook --> Lacie External HD --> Lacie External DVD Burner via FW all the time with no problems.