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There are a number of similar questions on AskUbuntu, but none of them describes exactly the same problem, and none has an answer that is easily adaptable (by me) to solve my problem.

PROBLEM

I had a successful Windows 10 / Ubuntu 20.04.1 installation running for some time. GRUB menu appeared on boot, with both Linux and Windows entries as expected. Both always booted fine. All good.

On booting today, I find that the Windows 10 entry in the GRUB menu has disappeared. I did not perform any action that might be expected to have this effect - just normal Windows 10 end-user apps, and occasional Linux usage. If I recall, there may have been a largish Windows update just before my last reboot. Not sure.

I have no means to boot to Windows.

I can boot to Ubuntu as normal via GRUB. From Ubuntu I can see and access the Windows partition and files on it.

QUESTION

There are several questions:

  1. Why does this happen? The fact that it happens spontaneously, with no obvious published solution, is especially concerning.
  2. How should I, and other users encountering this problem go about recovering access to the Windows boot process?

ATTEMPTS TO RESOLVE

sudo update-grub did not help.

I have installed boot-repair https://launchpad.net/~yannubuntu/+archive/ubuntu/boot-repair The report it generated is here: https://paste.ubuntu.com/p/YDJ3z44jhB/

It was unable to restore the Windows boot, but gave the following recommendation (bottom of the report):

Suggested repair: ______________________________________________________________

The default repair of the Boot-Repair utility would purge (in order to remove grub-efi) and reinstall the grub2 of sdb4 into the MBRs of all disks (except live-disks and removable disks without OS). Grub-efi would not be selected by default because: legacy-win no-win-efi Additional repair would be performed: unhide-bootmenu-10s win-legacy-basic-fix

Blockers in case of suggested repair: __________________________________________

LegacyWindows detected. Please enable BIOS-compatibility/CSM/Legacy mode in your UEFI firmware, and use this software from a live-CD (or live-USB). GPT detected. Please create a BIOS-Boot partition (>1MB, unformatted filesystem, bios_grub flag). This can be performed via tools such as Gparted. Then try again. Alternatively, you can retry after activating the [Separate /boot/efi partition:] option.

Final advice in case of suggested repair: ______________________________________

Please do not forget to make your BIOS boot on sdb (ATA WDC WD10EZEX-08W) disk! The boot of your PC is in UEFI mode. You may want to retry after changing it to BIOS-compatibility/CSM/Legacy mode.

I have changed from UEFI to CSM/Legacy using the UEFI menu. Running boot-repair still gives me the message:

LegacyWindows detected. Please enable BIOS-compatibility/CSM/Legacy mode in your UEFI firmware, and use this software from a live-CD (or live-USB).

Adam Smith
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  • Relevant but non-identical questions have been asked here...

    https://askubuntu.com/questions/398859/recovering-grub-after-installing-windows-7-with-boot-repair

    https://askubuntu.com/questions/390761/windows-7-doesnt-show-in-grub-menu

    https://askubuntu.com/questions/401629/windows-has-gone-missing-from-the-grub-menu

    https://askubuntu.com/questions/1150340/boot-repair-it-not-detecting-windows-and-hence-no-grub-menu-item-for-booting-win

     – Adam Smith Jan 20 '21 at 22:53
  • This question is similar at first sight: https://askubuntu.com/questions/197868/grub-does-not-detect-windows But OP was not previously running a successful dual boot, and the upvoted solution does not work in my case. – Adam Smith Jan 20 '21 at 22:55
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    Only boot in UEFI mode. You have UEFI Windows & UEFI Ubuntu, but Windows is missing its boot files in the ESP - efi system partition. It looks like entire /EFI/Microsoft folder is missing. You can easily repair using your Windows repair/recovery flash drive, but cannot repair from Linux. Some third party tools may work. UEFI boot entry for Windows shows that you at one time had /EFI/Microsoft folder. Nothing in Linux should have deleted that folder and I have not seen that with any other system, so unusual. – oldfred Jan 20 '21 at 23:15
  • Many thanks OldFred. So, it sounds as though it is a Windows issue, primarily, but one that can affect dual-boot users. I'll keep the question up if no one has any objections. Happy to make that the accepted answer if you move it out from the comments and no additional complications appear. – Adam Smith Jan 20 '21 at 23:23
  • This problem is probably the same: https://unix.stackexchange.com/questions/581491/problems-with-grub-and-windows-boot – Adam Smith Jan 21 '21 at 01:55
  • Ok. Finally back into Windows 10. Will try to add the steps, and clean up.
    1. Create widows installation media USB on second machine. I was following these instructions, but neither the basic nor more involved steps resolved the problem. https://answers.microsoft.com/en-us/windows/forum/all/restoring-windows-efi-files-without-wiping-linux/a2e46763-dac0-4541-bbc8-af0085cb67a9
     – Adam Smith Jan 21 '21 at 02:39
  • Followed these instructions using the command line available via the Windows installation media USB. https://superuser.com/questions/883809/fix-windows-8-efi-mbr-no-success

    • Bunch of error messages, but it rebooted into Windows 10.

    – Adam Smith Jan 21 '21 at 02:41