This might be something that users are aware of, but I wasn't and was bitten by it. Under OS X, and possibly other OSs, there is an issue with attaching multiple FireWire devices which are not terminated. I have an external Firewire HD attached to my PowerBook and had attached a Sony video camera to the second port on the back of the HD and the PowerBook was able to "see" the camera and download video even though the Firewire drive wasn't actually powered up.
The problem arose when I detached the video camera, but still had a Firewire cable attached to the secondary port on the HD. The next day, not realizing that there would be an issue, and frankly forgetting that the cable was still attached, I attempted to power up and access the HD. OS X 10.2.3 could find the drive but couldn't mount it and wanted to initialize it. "This is a problem," I thought. I had a lot of information I needed on that drive. I tried to access the HD several times, trying other things but nothing seemed to work. I booted into OS9 to see if it might be an OS X issue but the same thing happened: No external HD.
Then it occurred to me – didn't I still have that movie camera cable hooked to the drive. I pulled the cable off and OS9 gave me an icon. I booted into OSX and was able to see and access the drive with no difficulty. So, the moral is: make sure the Firewire chain is terminated with a device!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030116070134573