opps, just noticed your last line: "'ve replaced the cable, and tried an Amiga 2000 disk drive with the case open. Same problem."
so you have actually tried an alternate working floppy-drive! so it's not a floppy drive issue I guess. Perhaps the floppy controller chip is duff. Anyway, my above post maybe useful to any one who's reading this and having problems with a "dead" floppydrive or HD. So I'll leave it there. :-)