Thanks muchly for the welcome and for your advice!
Your point about the jumper raised doubts in my mind, too, so I removed the HD from its case, pulled/reinstalled the jumper (turned out I'd gotten it right the first time), and carefully pulled/replugged the IDE ribbon cable to the HD.
After I reassembled it and reconnected the USB cable to my computer, the setup immediately started working
My guess is, the IDE ribbon's connector was not well seated in the back of the HD (the ribbon gets very scrunched up when the HD is in place, and so has enough tension to pull out a partially plugged-in connector). Hope this experience helps somebody else down the road.