There’s a definite issue here. It’s not a loose cable or whether the machine is turned on or not. You can do anything you want and it won’t work except turn the power off and then unplug the Mac and start it back up. This issue is with a 3ghz Mac Pro in OSX and an HD Pro card. I have not seen it on a regular G5 or anything else. I had a similar issue with a Kona once and the thing is is the PCI slots still get power even when the G5 is off. By unplugging it you are doing a true reset of the card. It works perfectly after that.