Thanks, Alex, for the information. My problem was the latter: the iPhone was not showiing up in iTunes. What puzzled me was that Photos was detecting it and "reading" my iPhone. The matter resolved itself, which sounds crazy, I know. I can only think of a couple things that might have made the difference. I had originally plugged my iPhone into the USB port on the keyboard (not wireless). I have done that with previous iPhones without problems, but this time, it wasn't working.
I decided to unplug everything and restart my computer, and switch to my Dynex USB hub. After that all was well.
I suspect it was rebooting the iMac that set things straight, which is one of the things suggested in the second link you provided.
Thanks for your help.