A small number of users lost the use of their older FireWire 400 disc burners (and perhaps other FireWire peripherals) after installing Snow Leopard. I have a fairly new Pioneer DVD burner installed in a circa-2001 LaCie enclosure, and I was one of the victims. The 10.6.1 update did not address this issue. Until Apple remedies the situation, here is a solution that works without any obvious side-effects:
Locate IOFirewireSerialBusProtocolTransport.kext in /System/Library/Extensions, and store it in a safe place. Replace this item with the same extension from Mac OS X 10.5.x Leopard. The date stamp on my copy was October 9, 2007. (You may still have a copy in your Time Machine backup.) After replacing the file, use Disk Utility to Repair Permissions. This step is vital, or else the replacement extension may not be recognized by the System. Finally, restart your Mac.
After the restart, if your FireWire drive worked properly under 10.5.x, it should work fine again.
NOTE: This hint involves mucking around in your System folder. Appropriate precautions are in order. Do not perform this operation if you are not experiencing issues with your FireWire devices. I did not originate this hint; it came to me as an anonymous reply to a message I posted to several places on the 'net.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090922213608468