I'm having this very same problem with my 500 GB My Book and 1.66 GHz Core Duo Mac Mini. It worked perfectly fine over firewire before the upgrade and for a few days after the installation, until one day I went to start iPhoto, and I was told that my mac couldn't find the external hard drive on which my photos are stored. I've tried all of the fixes mentioned here, and nothing seems to work. I've tried USB instead, Path Finder, plugging the power cord directly into the wall, and now it's not even showing up when plugged into Windows computers via USB.
Any other suggestions?