I’m posting this in case anyone else runs into the same problem. It will save hours/days of frustration.
I tried to image the other night but my camera (D5100) wouldn’t connect to SGP - or any other of my astro programs. However, it connected to my laptop and I could view images from my SD Card. In fact my laptop and camera worked fine in all my other tests. I won’t go into everything I tried, suffice to say I tried a LOT of things.
The problem was caused because I had mounted an ISO file in Windows 8.1. This is a feature that allows you to execute an ISO file from within windows. I have been using it extensively to install Windows 10 for myself and others. In this case I had mounted it on my astronomy laptop with the idea of installing it later - after I had done more testing with windows 10. It seems leaving it mounted without executing it caused the problem. Easy to fix by ejecting it. Everything now works again. 