Can't get into Windows10 from GRUB2; boot-repair isn't helping

fourOfour

5/9/24, 5:26 PM

Good afternoon,

I'm aware there's several questions like this here, but as none of them answered my problem and as I am utterly and completely lost, I figured I might as well try here.

My system information is described in https://paste.ubuntu.com/p/JNnVJ3Ft5D/, but in short: separate FAT32 sda2 boot partition at the start of the disk (with boot and esp flags, despite the partition name), followed by an sda1 Lubuntu 22.04 partition and an sda3 Win10 one. Only Lubuntu shows as an option on startup. os-prober isn't disabled in the GRUB config files, and the pastebin lists Win10 as an OS normally, so I very much have no idea of what's going on.

I tried running Windows Startup Repair via a rescue disk - I knew that'd uninstall GRUB, but I figured I could just reinstall with boot-repair immediately after - but no go. boot-repair itself hasn't been much help either, it keeps telling me "Legacy Windows detected, you might want to disable the separate boot partition", but then if I do the program instead tells me "GPT detected, please create a partition at the start of the disk like X and Y and Z, or alternatively turn the option for the separate boot partition on".

The pastebin does mention "no ESP detected" in several places, so I thought that maybe if I add the ESP flag to the FAT32 boot partition that'd help things, but apparently that does nothing?

Any and all help would be greatly appreciated.

EDIT: a commenter (@oldfred) mentioned GRUB being installed to MBR, which does seem like it could be the issue, but I don't know how to uninstall it from there. Just to make sure, I tried running boot-repair again with the purge-grub option on, but ran into the same issue of "Legacy Windows detected" yada yada (and I can't very well turn off the option for the separate partition). The corresponding BootInfoSummary is here, but nothing seems to have been done other than a regular reinstall.

Also, to be clear, all this has been done exclusively in UEFI mode. (Pretty sure boot-repair doesn't work outside of it anyway.) Windows is listed as the OS for sda3, the Windows loader files seem to be right there in the Windows system folder, and really nothing seems to be wrong - it's just that the Windows loader simply doesn't show up as an option in the GRUB menu.

0 + 3

partitioning

boot

grub2

dual-boot

oldfred

5/9/24, 9:21 PM

You have UEFI system and should only have UEFI installs. But have grub installed to MBR when it only should be in ESP - efi system partition. It looks like you converted the fAT32 ESP to bios_grub. ESP must be FAT32 with esp,boot flags. The bios grub should only be 1MB unformatted with bios_grub flag. Hopefully setting bios_grub on ESP did not erase files, but even if it did, you just need to reinstall grub in UEFI mode & from Windows repair/recovery flash drive in UEFI mode reinstall Windows boot loader. First Ubuntu entry has wrong GUID/partUUID, second one looks correct.

fourOfour

5/11/24, 2:46 AM

This is insightful, but I'm not sure what to do to correct it. The ESP partition (sda2) should have the correct flags and formatting. I still run into the same problem as before, though - "Legacy Windows detected, you might want to disable the separate boot partition" - and I can't very well disable that option. To make sure, I reinstalled GRUB in UEFI mode (boot-repair doesn't even run in BIOS compability mode anyways afaik) and got another BootInfoSummary out of it, which can be found [here](https://paste.ubuntu.com/p/GnYbbX6Dyt/). I really have no idea what's wrong.

oldfred

5/11/24, 7:13 PM

Boot-Repair is looking into your ESP for Windows boot files. Not finding any, it thinks you have a BIOS/Legacy type install. But you have an old UEFI boot entry as shown with efibootmgr -v and it uses a GUID/partUUID that does not exist on your system (old deleted partition). You need to use Windows repair tools booted in UEFI mode, as Boot-Repair does not fix Windows issues. Windows also only boots in UEFI mode from gpt partitioned drives, so you must have an UEFI install of Windows.

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