Geek Post- Leopard Weirdness
Not sure if it was a hardware or software problem, I backed everything up and reloaded Leopard using the erase/reload method, and the problem remained. Uh oh. Then, primarily because I'm lazy, I grabbed the Tiger install disks that came with my recently purchased Mini. Turns out, they won't load on the iMac. They don't say why- just "cannot be installed". Ugh? The Leopard upgrade disk that I used on the iMac was from the up-to-date Apple program (which I qualified for due to the Mini) and I'm now wondering if it is the same Mini-specific deal. It never errored or complained, but who is to say it wasn't the wrong tool for the job.
So then, I lumbered downstairs to get the iMac's (re)install media and loaded it, and hey presto! The firewire worked again. I know now that it's not a hardware error, as all devices are humming along at present. I figure later tonight, I'll upgrade to Leopard again using my non-upgrade family pack media and see what happens. At minimum, it appears that the up-to-date "upgrade" disk that was intended for a Mini has issues with the iMac. Perhaps more alarming, there may be an issue with the Firewire on Aluminium* iMacs.
All media are not apparently, created equal(ly).
* No, sorry. You're wrong on this one. "Aluminum" is NOT the correct spelling of element #13. I don't care what the Blogger spell check thinks. "Aluminum" is a bullshit American bastardization of the word (and you can correctly infer from my spelling of "bastardization" that I am an American). It's not "Ah-loom-ih-numb", it's "Ah-loo-min-ee-um", or "Al-you-min-ee-um" if you are in the UK. Yes, I realize that it's the product of debate between the Brits and Yanks, but they happen to be arguing in favor of consistency (see CesIUM, PotassIUM, MagnesIUM, CalcIUM...), a quality often lacking from this language. You say you prefer the "-um"? Well, you're in great company, there with Molybdenum... *snicker*
After considerable testing of various permutations/formats/reloads, I have isolated the core issue. If you have an Aluminium iMac (and potentially others) and boot up the system WITHOUT any Firewire devices attached and powered on, then the Firewire 400 port will be either disabled, or hard set to the Firewire 800 interface (nothing to test the FW800 with). However, if you power up with a Firewire device at the ready, then once it's booted, you can mount and unmount at will. This does not happen with Tiger- only with Leopard. Weird.