Grub drops to prompt after mkconfig finds Win10 [SOLVED]

audiodef · Last edited by audiodef on Mon May 07, 2018 11:27 pm; edited 1 time in total

I just set up a dual-boot with Win10. In chroot, grub-mkconfig would not find Win10. (os-prober is installed.) But after booting into my fresh Gentoo install, I could grub-mkconfig again and Win10 was found. But thereafter, Grub dropped to a prompt. I fixed it by chrooting back in and running grub-mkconfig again, where Win10 was not found by Grub.

Until I fix this, I'm using the BIOS to boot into Win10 when I need it.

So it seems something goes wrong when Grub picks up Win10, although no error is reported. What's happening and how do I fix it?
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DONAHUE · Posted: Mon Mar 19, 2018 9:45 pm Post subject:

Are either or neither or both Win10 and Gentoo installed as UEFI?
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audiodef · Posted: Tue Mar 20, 2018 12:53 pm Post subject:

Both installed as UEFI with the laptop in full UEFI mode.

I forgot to mention - if this is important - that as an experiment, I used the EFI partition that Win10 set up during its own install. I figured it should work, since creating another EFI partition for Gentoo would simply result in a second EFI partition with exactly the same fs anyway. There is plenty of space on this partition to hold both Win10 and Gentoo EFI stuff.
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DONAHUE · Posted: Tue Mar 20, 2018 2:52 pm Post subject:

"I used the EFI partition that Win10 set up during its own install." That is absolutely the right thing to do. Multiple EFI Systems Partition's (ESP) can cause confusion.
It appears that grub installed on the ESP but can't find or can't read grub.cfg.
Are you sure Windows is behaving? boot windows, Press Win + R > type msinfo32 > press Enter. BIOS Mode entry should be UEFI
Are you sure that the installed Gentoo system was in UEFI mode when you were running grub-install? Boot installed Gentoo. and run

audiodef · Posted: Tue Mar 20, 2018 4:45 pm Post subject:

DONAHUE,

Thanks for your help. I'm glad I was right about using the existing EFI partition.

Windows is behaving. My install appears to be in UEFI mode. (It should be, no legacy options were enabled in the BIOS when installing both Windows and Gentoo.)

ls -lR /boot | wgetpaste -t: https://paste.pound-python.org/show/Chfh8syMEvk1zYKgwLHN/

Using a separate boot partition, but EFI is on the boot partition as /boot/EFI.

fdisk -l | wgetpaste -t: https://paste.pound-python.org/show/lz6lz87frCO0l0gjszam/

mount | wgetpaste -t: https://paste.pound-python.org/show/vPkqvXkYaLhBCHCpBZ6D/
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DONAHUE · Posted: Tue Mar 20, 2018 8:08 pm Post subject:

Do you have to mind the exact command options used when you did grub-install?
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audiodef · Posted: Wed Mar 21, 2018 2:02 am Post subject:

I did

bammbamm808 · Guru Joined: 08 Dec 2002 Posts: 548 Location: Hawaii

If you get sick of spending do much effort configuring software you might not need, consider the rEFInd boot manager instead.
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audiodef · Posted: Wed Mar 21, 2018 1:34 pm Post subject:

I was reading about using EFI itself as a bootloader, but I'd been haxxing with this laptop for several days, figuring out what dual-boot and config options I want, so I decided I wanted to stop haxxing and start using. Next time I set up a UEFI machine, though, I'm going to learn how to do this.
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skellr · Posted: Wed Mar 21, 2018 2:59 pm Post subject:

DONAHUE · Posted: Wed Mar 21, 2018 4:38 pm Post subject:

The ls -lR pastebin earlier shows "grub-install --efi-directory=/boot" worked well as far as placing correct files in the correct places on the ESP. (also known, in this case, as the boot partition).
If I grasp the problem correctly: booting installed Gentoo, mounting /boot, running grub-mkinstall -o /boot/grub/grub.cfg (with os-prober installed) produces output resembling mine

skellr · Posted: Wed Mar 21, 2018 5:00 pm Post subject:

audiodef · Posted: Thu Mar 22, 2018 12:36 pm Post subject:

What's happening is this:

emerge -C os-prober (don't find Windows, leave it out)

grub-mkconfig -o /boot/grub/grub.cfg

Output as expected, succeeds

Reboot

Grub appears normally, boot into Gentoo

Now...

emerge os-prober (let's find Windows and include it)

grub-mkconfig -o /boot/grub/grub.cfg

Output as expected, succeeds, Gentoo and Windows written to grub.cfg

Reboot

Grub drops to prompt

If I emerge -C os-prober and redo grub-mkconfig, grub goes back to showing boot options and booting normally into Gentoo.

So the question is: why does this work when Windows is ignored, and grub drops to prompt on reboot after adding Windows to grub.cfg?
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DONAHUE · Posted: Thu Mar 22, 2018 4:10 pm Post subject:

Is there any terminal output before the grub prompt appears?
Can you pastebin a grub.cfg that includes windows and one that does not?
Suggest print the windows containing grub.cfg. Then boot it and enter the windows menu commands manually at the grub prompt. Record any output from the commands.
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audiodef · Posted: Mon May 07, 2018 11:27 pm Post subject:

Finally got back to this issue and solved it (experimentally, not on purpose) by keywording os-prober and adding the "mount" use flag to grub. After that, grub-mkconfig simply worked as expected and I can choose Windows or Gentoo without having to go into the BIOS to make this choice.
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