After my install on Friday none of my external hard drives showed up. I had a computer discussion with an Apple rep and no luck, on Saturday I had a telephone call with an Apple rep and no luck, but I did find out, using Safe Mode, that the computer could read the external hard drives as before. In normal mode it still did not read them. The Apple rep was going to have a discussion with a "higher-up" and get back to me on Tuesday. I continued on to test this and that without luck, then unplugged my Wacom tablet and the hard drives all showed up! Apparently it was the driver on my Wacom tablet. Both Apple reps were very nice and tried hard to help, it was just luck that I discovered what it was. So, beware of added things when you update and suspect the driver if there is a problem. It's always good, of course, to assure that every plugged in item is updated and that all software is current prior to updating...a lesson learned. All is working fine now as before.