Thanks for the ideas!

But I investigated further, and it appears that I really do have two floppy drives which produce exactly the same sort of error messages. If I install the suspect drive into a different computer, I see  the exact same problems there too. And if I install a known-good drive into the Portable, it works flawlessly. I guess it's a relief that it wasn't my circa 2008 re-cap job which was causing problems.