I had the same problem. There is a dialog somewhere at the start of the install process that gives you the option to install Windows 10 without updates. I tried installing again, this time i chose to install Windows 10 without updates. It worked for me. After my computer rebooted, updates started installing automatically anyway. There was a minute when the screen went black, so if that happens to you don't freak out. It will come back on. I hope this works for you.
This worked for me! Thank you so much. For those of you who want to try this method, here is what to look for:
When you are upgrading to Windows 10, one of the first screens that you see is titled "Install Windows 10". This has a paragraph explaining updates. Look below this paragraph for the link (in blue letters): "Change how Windows Setup downloads updates". Click on this link.
This brings up a screen titled "Get update, drivers and optional features." Select the "Not right now" option.
This way you can do the upgrade without first downloading the updates. If the upgrade is successful, then the follow-up updating can take place. In my case this updating began automatically, and it took quite a while, more than one hour.
I was having the same issue, i.e., the installation failed at 73%. I spoke with Tech Support at least 4 times and never got anywhere but one of their techs provided me the URL to this Community group. And there was the solution as noted above by Howard White1. However, when launching the download file, MediaCreationTool21H2.exe, that prompt was not provided. So I launched the installation from the setup.exe file on a USB media I had created and there was the option. Interesting. Anyway, Windows 10 installed over my 8.1 laptop and things are rolling right along...till the next release. Thanks, Howard!