From the sounds of it, you're plugging in your burner while your computer is up and running. If this is the case, what you're doing is known as hot-plugging it, which SCSI doesn't really support. It may have worked before in OS9, but OSX is probably less tolerant of this, plus you can cause damage to your computer or burner. Try plugging it in before you boot your computer. You'll have to leave it turned on all the time as well.