Partial breakthrough, but it's making me question my sanity even more...
Drive now works with 100% perfection, as long as I do NOT have a keyboard plugged in.
Doesn't matter WHICH keyboard (I've tried 3), nor does it matter if I plug the keyboard into the hub or the Pi itself.
And once the Pi is booted, drive running perfectly, if I plug in a keyboard, the drive instantly disconnects.
So either I'm going insane, or the drive's USB/SATA interface is bad in some wonky way.