After attempting around ten installs of the new Windows 10 build the other day, and reading many forums posts, I've finally got it working! The posts varied somewhat, but mostly seemed to center on devices being the problem. Like disconnect all external devices, or run the upgrade from Safe Mode. I tried everything I could think of, until I noticed one thread mentioning free hard drive space. I had under 20GB free on my drive. I spent some time and got it to 50GB, and low and behold the update worked! I'm not on the latest build. Finally!
If you keep getting the 0xC1900101 error with the build upgrade failing, FREE UP DRIVE SPACE!