I am using my Lenovo T420S now since more than four years. This notebook is still comparable to current machines that are now on sale. It is equipped with a I7 processor and 16GB of RAM. In addition I added two Samsung SSDs during the years. I haven´t found any significant better notebook of that quality level since I started to look for a new one in this summer. So I decided to still stay with the T420 since there is a lot better one on the market that is worth its price. Best on that decision I had to trying to get Windows 10 working on that machine.
After several (> 10) tries I was finally able to upgrade to Windows 10. Me was confronted with an always occuring 0xC1900101 – 0x20017 error, after the system rebooted for the first time during installation (before the actual upgrade starts). In my case, the T420 rebooted and a start up screen with the Windows logo is shown. After a couple of seconds a single white dot was shown in the lower middle of the screen (should have been a loading spinner). After rebooting, Windows 7 is restored and the following error code is shown: