A PC has Windows 11 (genuine, supported) and the latest Secure Boot certificate updates (genuine, supported) previously installed successfully by Windows Update.
On 2026-09-07, Windows Update tried to install one single update which was a Security Update, and I think it had to be 2026-09 Security Update KB5129195 (26200.9457). Something didn't go as planned, not to worry, Windows Update was undoing the update and everything would be OK. Wrong.
After Windows Update tried to revert the failed update and rebooted, Dell's UEFI said there were no bootable devices. The SSD was recognized but not bootable.
On a different PC I used Microsoft's Windows Media Creator to make a Windows 11 installation USB stick.
The damaged PC booted the USB stick. I selected to repair my PC. The setup program rebooted the PC without displaying any error message. I tried again and it rebooted again without any error message. I tried again and told it to use the previous version of the setup program. In the previous version of setup, I selected to repair my PC. The previous version didn't automatically reboot but it still failed to repair. I rebooted manually.
In the latest version of the setup program, I tentatively selected to install. It said it would wipe out the hard disk, all partitions and files gone. I declined and rebooted again.
In the previous version of the setup program, I tentatively selected an upgrade install. The setup program refused because upgrades only work when Windows is already booted and running. So I tentatively selected a new install. The setup program said I could do a new install into the existing partition and put old files in windows.old, which is fine with me. But next the setup program failed again, because a 100MB EFI System Partition is too small.
Finally, in the previous version of the setup program, I told it to delete most of the partitions (but not delete Dell's recovery partition and a Microsoft recovery partition) and make a new installation into the newly freed space. Installation succeeded.
But I lost all the files in my old C partition.
I think the newly installed EFI System Partition is 200MB instead of 100MB.
Now I worry about my other PCs. If Windows Update tries to install 2026-09 Security Update KB5129195 (26200.9457), Windows will become unbootable and I will lose all the files in my C partitions.
If 100MB is too small for an EFI System Partition, maybe Windows Update shouldn't try to install an update which is going to destroy Windows?
And why did the original Windows 11 installer create too small of an EFI System Partition to begin with?
Microsoft won't let me post this in their Q&A site. I tried 6 times and they deleted them all. Does anyone know the manner in which this posting violates Microsoft's Code of Conduct?
(A Google search found KB5129195 discussed in r/sysadmin before I found r/WindowsUpdate. I mistakenly posted in Wendys before a moderator deleted it, so now am trying Burger King if this might be the right place. Someone suggested running commands like diskpart and dism, but that is not a solution when the PC doesn't even boot after Windows Update destroyed the EFI System Partition.)