WOOHOO!
I think I finally got it. Turns out to have been a bad or incompatible IDE cable. Thought I had already tried swapping them, but I guess not.
Whew. Thanks again for the help, Rosie.
Yay! Glad it turned out OK. And it’s all good information to file away...
Hope I actually was of some help!
"I was just getting ready to suggest that when I read you'd already done it," he said nonchalantly...