Humm, I have the same drive in a different Firewire case and it works without any problem at all.
I've sold the same drive to others and no problems from them either, nothing but raves.
Do you have the lastest version of Diskburner?
Have you tried using Toast, rather than Apple's diskburner? Personally I prefer Toast for burns, but then I have no problem with DiskBurner like you're describing anyway.
If you have access to another system, check and see if the drive burns fine on that with the same setup. If it works fine, then you can probably be certain its something with your system. If it does the same thing, then possibly either the drive mech, or the Firewire bridge in the external case have a problem.