SOLVED: Poor performance with USB disk drives

I was getting extremely slow backups and backup failures with USB disk drives of different brands. Cure was to (1) disable the motherboard USB ports, and (2) use new USB ports on a card. Crucially, step (1) was necessary and (2) didn't help without it. Details:
http://www.covingtoninnovations.com/michael/blog/1007/index.html#100721A
in hopes of saving other people the same struggle!