In the past ten months, I had failed to upgrade my older computer to Windows 10 numerous times.   Various google-searched solutions were tried but futile in the end.  The upgrade process always stopped at 20- or 70-somethings and reverted back to the original OS.  It was quite frustrating.

Last weekend, I was finally successful to upgrade the computer to Windows 10.  The optical drive was the key!  

Before starting the upgrade process, I unplugged the DVD drive as well in addition to all other extra storages and USB devices.   It was not that I was lazy not to unplug it in all previous attempts.   I was just simply not aware that the optical drive was also a critical factor in dealing with the upgrade issue.

Worth a try if you are facing similar Windows 10 upgrade issue.