I want to add a external hard drive and wireless n usb dongle, but the G5 is short of usb ports.So I got a usb 2.0 extension card and installed it into my G5, and this happened: http://www.youtube.com/watch?v=KWLoKSu1aY8 (Not my video but mine behaved exactly like the machine in the video) Removing the card immediately solves the problem, I also tested the card in a PC and it worked fine. I also hooked up an external hard drive to the extension card, it powered on briefly before the G5 shut down. http://youtu.be/MuTd6mIX63M
Check the chip on the USB card, if it is not NEC then you are out of luck. NEC chipped USB card are the most Mac compatible. I have never failed in one.
I did "reset-all" and "reset-pram" in open firmware then try to install the card again. Didn't work either