Here's my latest adventure.
I did not touch my XP/Firewire setup, as I dont' want to break anything, since it was burning at full speed (8x peak and 6x average on 8x disc). I installed Nero and ImgTool on my Win2k/USB2 desktop. It failed with "SCSSI Command Aborted". I then tried every possible freeware burner, and finally found one working with DeepBurn. It was able to burn both on my Win2k/USB2 P3 desktop, and Win2K/USB2 P4M laptop. However, it was only able to do so at 2x. The setup also requires me to unplug my various other USB accessories (I had a ton).
Pioneer's site said A09XLB could only burn 4x, 6x, and 8x on 8x disc. That's probably why most other software failed. But DeepBurn started with 8x, and somehow able to settle on 2x in the end. Hence, it was able to complete the burn. All in all, I'm able to use my A09XLB on all three of my computers, although not at a speed I wanted. Guess I'll just use the XP/Firewire setup for regular burning, and the other two for occational backups.
In summary, I conclude 1) A09XLB works better with Firewire; 2) when it fails, try a differnt burning software instead of playing around with system setup, since there isn't much you can change when you are connected externally.; 3) for USB2 connection, reduce other USB activities as much as possible (unplug the unused ports).