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:

The installation failed in the SAFE_OS phase with an error during BOOT operation.

To avoid a long try and error marathon for you, you just might have to follow the steps I am listing below. It might be, that not all of the steps are necessary to perform. I am sorry, but I tried several steps with each installation attempt. So, you can just check, if a subset helps you.

  1. Use the windows media creation tool to download the Windows 10 upgrade directly to an USB stick from which the installation can start later on.
  2. When installation starts and you are asked if you would like to update the installation software, you should do it.
  3. Finally I went to change some BIOS settings and those are the ones that enabled me to install Windows 10.
    1. Config > Display
      1. Boot Display Device: Thinkpad LCD
      2. Graphics Device: Discrete Graphics
      3. OS Detection for NVIDIA Optimus: Disabled
    2. Config > Power
      1. Intel Speedstep technology: Disabled
      2. CPU Power Management: Disabled
      3. PCI Express Power Management: Disabled
    3. Config > CPU
      1. Core Multi-Processing: Disabled
      2. Intel Hyper-Threading Technology: Disabled
    4. Security > Memory Protection
      1.  Execution Prevention: Disabled
    5. Security > Virtualization
      1. Intel Virtualization Technology: Disabled
      2. Intel VT-d Feature: Disabled
    6. Security > I/O Port Access
      1. All to disabled, expect of Ethernet LAN and eSATA Port to update the Windows installation software before the update
    7. Startup
      1. UEFI/Legacy Boot: Both
      2. UEFI/Legacy Boot Priority: UEFI First
      3. Reserve memory for UEFI Boot Manager: Enabled

With theses settings in the BIOS I was now able to install Windows 10 to my machine. After the installation finished, you can set back all settings to their original state, expect the Config > CPU > Core Multi-Processing that needs to stay in the state „Disabled“.

From my current point of knowledge, the Core Multi-Processing is not effecting the actual active cores of the CPU, but is just some kind of „combining“ multiple cores to process a single thread. On my notebook this isn´t affecting the speed of the machine at all.

After installing Windows 10 I recommend to install software updates from Lenovo to your system. There are for instance new drivers for the NVIDIA Optimus.