The other day, I connected an external firewire drive to the front firewire 400 port on my G5 dual 2.0 GHz as I have done a zillion times before. There was an audible pop, and the whole system shut down. I waited a couple of minutes, and then restarted, got all the way to log in, but kernel panicked as the desktop (and connected firewire devices) came up. So, shut down completely, removed all firewire devices, 800 and 400, from the computer, and powered it up again. All hunky dory, no issues, steady as a rock.
Checked in System profiler, and it showed the firewire bus as still viable. So, connected one device on the 800 port, and yet another kernel panic. Obviously, the firewire bus was blown and unusable. Bit of a problem as I have 3 external drives, DVD burner, EyeTV, etc., permanently connected and can't function without them. According to my friendly local Apple repair centre, I needed a new logic board to restore the original bus -- system more than 2 years old and no AppleCare, so huge cost; not an option just now.
Instead, I bought a Sonnet FW800 PCI card and a couple of firewire 800-400 adaptor cables for about £100. It arrived within 24 hours, and 20 minutes later, all my drives and other bits of gear were all connected up and running perfectly once more.
According to the Apple repair centre I use, it's a common problem on the G5s. Having it blow on my own system was the first I'd ever heard of it! Just wondered if anyone else here has had a similar experience.