Well, I'm going to exhume this old topic just for future reference for users who don't want (just like me) upgrade Toast.
Today this same error almost drove me mad: firmware was updated, no major update to my system (Tiger 10.4.11 PPC), changed cable, tried Burn that gave me too an error like "the unit is not responding"...
Just tried with a different media: always TDK -R, but different media code... and everything ran smooth.
I strongly advise not to buy TDK DVD -R with media id MBI 01RG40; see
here