After many, many failed attempts, I finally upgraded the last computer that I use regularly to Windows 10. I’m not certain what the issue was; but I suspect lack of disk space. The Windows.old folder is HUGE. Much larger than on any of my other systems. It certainly would have been nice to receive an error message to that effect. Especially since the program theoretically checks to ensure enough disk space is available. The setup logs were amazingly uninformative. And, since everything gets deleted, upon failure, there was no way to tell what that problem was. It was coincidence that I had more space available this time around. What a gigantic pain.