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Jeudi, 6 février 2014

Changing GRUB from EFI to BIOS

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Catégories : [ Informatique ]

My new computer has a UEFI firmware. I installed Debian Wheezy, which in turn installed the EFI variant of GRUB. For that purpose, the Debian installer made the first partition on the hard disk drive of type VFAT and mounted in /boot/efi.

My problem is that GRUB tends to freeze, either just before booting the kernel (showing forever “Loading initial ramdisk”) or just after the welcome message (“Welcome to Grub!”). Pressing the computer's reset button allowed to reboot the computer, and everying went then fine. It seems to be possible to reproduce the bug at will by switching off the power supply, waiting 15 seconds for the capacitors to get empty and then reboot the computer. Booting however also hangs quite often after powering the computer off in software (where the power supply still provides some power to the motherboard).

I read here and there that EFI GRUB was quite buggy, so I decided to switch to PC GRUB (the variant for booting with the Legacy firmware, aka BIOS).

In a first attempt, I configured the motherboard's firmware to use “Legacy ROM only” instead of “UEFI only”. Debian continued to boot normally with the still installed EFI GRUB, and the freeze when rebooting after having switched off the power supply seemed to have disappeared. It howerver froze again today and so I decided to change from EFI GRUB to BIOS GRUB.

I first ran apt-get install grub-pc, which complained that
/usr/sbin/grub-setup: warn: This GPT partition label has no BIOS Boot
 Partition; embedding won't be possible!.
/usr/sbin/grub-setup: warn: Embedding is not possible.
 GRUB can only be installed in this setup by using blocklists.
 However, blocklists are UNRELIABLE and their use is discouraged..

After a bit of research on the Web, I found someone's advice to change the flag of the FAT partition to bios_grub. I then forced the reinstallation of grub-pc with apt-get install --reinstall grub-pc, which didn't complain anymore.

On the next reboot however, the startup script indicated that “fsck died with status 6”. I found out that it tried to check the VFAT partition, but since GRUB is now installed there, it is not anymore recognized as a VFAT partition, and fsck was legitimately skipping it. parted confirmed that fact, and blkid does not list the VFAT partition anymore either. I therefore commented it out in /etc/fstab and now the boot does not fail anymore.

[ Posté le 6 février 2014 à 19:23 | pas de commentaire | ]

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