Hmmm... I have an external Lacie burner which works fine under Leopard. I assume you have updated the drive with the latest software updates? I would also recommend making sure you are running Apples latest 10.5.4 update as well. The next thing I would do is trash the devices pilst (which is found under finder-user-library-preferences), and try again.
What happens if you put a cd or DVD in the drive?
Next up..With the devices plugged in, go up to the Apple menu and select about this mac-more info-disc burning. If it lists the drive as "unsupported" I'm not sure what you can do next.
I'll poke around the web and see If I can offer any more suggestions.
Mark
