Thanks for the prompt replies.
Actually, I think I have resolved my problem. Before I could implement your
advice, I barely bumped the USB hub and the power failed. That led to a
corrupt SD card, and time spent rebuilding to the point where I could try
setting up to run my Pi from an ext HD. Once I replaced what was obviously a
flaky USB Hub, the problems I described in my first post went away.
So, again, thank you all so much for your attempts to help me. I do think,
however, that it was all down to a faulty USB Hub. For anyone who has similar
issues, if you can, swap out your hub, just to be sure that's not the problem.
The Raspberry Pi Hobbyist: Running From an External Hard Drive
http://raspberrypihobbyist.blogspot.co. ... drive.html)