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I just upgraded my HP zBook to Fedora 34. After the upgrade, Grub booting failed. The machine now just boots into a plain Grub prompt

grub2>

I can't see any error message during boot, maybe something is shown but is immediately flashed and all I see is the above prompt.

When I enter the following sequence

set prefix=(hd1,gpt5)/grub2
set root=(lvm/fedora_localhost--live-root)
insmod normal
insmod linux
normal

then the usual Grub menu appears, with the new Fedora 34 kernel as the default and everything works as expected. So the upgrade as such was successful.

I suspected that somehow the boot sequence might have changed. I have 2 disk in the laptop (a Samsung SSD which is visible as /dev/sda with a DOS partitioning and a WDC which is visible as /dev/nvme0n1 with a gpt partitioning). When I try to re-create the grub.cfg, the following error message is shown

# grub2-mkconfig -o /boot/grub2/grub.cfg
Generating grub configuration file ...
/usr/sbin/grub2-probe: error: ../grub-core/kern/fs.c:120:unknown filesystem.

and the resulting grub.cfg looks fragemented to me

# ls -l /boot/grub2/grub.cfg
-rwx------. 1 root root 143 13. Mai 22:09 /boot/grub2/grub.cfg

Happy for any suggestions how to get out of this again. Obviously I googled a lot and tried various hints, but nothing has worked for me so far

CU, Joe

1 Answer 1

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Reading many more articles, it seems that running

grub2-mkconfig -o /boot/grub2/grub.cfg

is actually supposed to fix this issue. GRUB2 2.06 seems to have known limitations that prohibits successfully running this command (as in my case, with the error message shown above). The suggestion was to downgrade to GRBU2 2.04, which should then allow to run this command

I didn't find any easy direct way to downgrade to GRUB2 2.04, so I ended up download all required RPMs from here https://koji.fedoraproject.org/koji/buildinfo?buildID=1668414 and then installing them with

dnf install grub2-*.rpm

After that, indeed, the grub2-mkconfig -o /boot/grub2/grub.cfg ran without any issues and with the next reboot, the usual GRUB menu appeared again during boot

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