Windows 11 upgrade error: 0xC1900101 – 0x20017 — the installation failed during the SAFE_OS phase

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Note: English-language articles on this website are usually machine translated.

Don’t let it send you down the wrong path! If you dump the setuperr.log or other upgrade logs into ChatGPT, for example, or even straight into Google, you’ll usually be sent off on a long, drawn-out hunt for outdated drivers or LowerFilters buried deep in the registry from some old CD-burning software. Stop right there! That can be the cause, and I’m not saying there’s no value in dissecting the log files.

But after several months of experience with Windows 11 upgrades, one thing has become clear: in 90% of these cases, the real issue is a faulty EFI partition that prevents the upgrade. This does not show up clearly in the log files. There are plenty of errors listed, but none of them point to this underlying problem.

For background: your EFI partition may appear to work perfectly fine. Your Windows 10 system boots without any issues under all normal conditions. The UEFI/BIOS correctly detects the “Windows Boot Manager” on the EFI partition, and everything looks fine. But something is still wrong with that EFI partition — something serious enough to cause the upgrade from Windows 10 to Windows 11 to fail at a very early stage, even though Microsoft still lets the entire process run through, only to roll everything back at the end.

Solution: recreate the EFI partition. Completely deleting it is not necessary, but formatting it and repopulating it with the boot files/BCD is.

Please read through the following steps completely before carrying them out! They include warnings that you should take seriously so you do not format the wrong partition or end up with a system that will no longer boot afterward. Use this procedure entirely at your own risk!

Boot into Windows RE from the ISO image, ideally a Windows 10 22H2 ISO, and then open the Command Prompt.

In the following example, the system partition containing the “Windows” folder is actually C:\ in the recovery environment — please make sure to check first whether that is also the case in your Windows RE.
diskpart
list disk (identify the disk that contains the EFI partition, often Disk 0)
select disk 0
list vol (identify the EFI partition; it is usually 100 MB or 260 MB, GPT, and formatted as FAT32)
select vol 3 (used as an example here)
format fs=fat32 quick (make sure the correct volume is selected before running this command!)
assign letter=U: (used as an example here; any available drive letter will do)
exit
bcdboot C:\Windows /l de-de /s U: /f UEFI

That covers the basics. Here’s the advanced part, in case you also modified the recovery partition — or plan to — and reagentc.exe /enable returns error 3B3C, or winre.wim cannot be found: Error 3B3C while trying to enable the recovery partition using reagentc.exe

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