Macbook Pro boots with a white circle with a line through it.
Others have identified this as a machine with the wrong OS on it, an incompatible version of the OS on a Mac. However, I have found another cause. To resolve the issue if it occurs, if it is caused by circumstances like mine, you would hold option+command+R then go into recovery and choose time machine and roll back. Then try again. In my case I saw the Mac had updates. I selected to do the update which downloaded a 1.2gb file. During that download process I noticed that this mac could have Catalina installed (a new version of the OS). During this download the updater indicated that it wanted to reboot. I cancelled that. I let the Catalina downloader (no it hadn't installed anything, it was just downloading) go for…