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I recently upgraded my quite old (I think installed around 6 years ago, and upgraded a few times since) installation a few weeks ago. When rebooted it won't boot up with the following grub error:

'error: unknown filesystem'

Upon which it enters rescue mode.

I'm assuming that the issue was introduced by the upgrade, but I can't be sure as the system had been up months before the upgrade.

It's pure Ubuntu. Never been anything else, or dual booted etc.

The install disc has 3 partitions, sda1 / ext4, sda2 swap and sda3 'everything else' - a storage partition mounted under /media which is ext3.

I have a feeling that sda1 / was ext3 before the upgrade, but can't be sure, I don't pay attention to details when they work...

ls (hd0, msdos1) and (hd0, msdos2) returns 'Filesystem is unknown'. ls (hd0, mddos3) returns ext2.

I've booted Gpartd and checked the filesystems. They're all fine. /dev/sda1 is ext4 and has 'boot' flag set, and when mounted looks ok to me. /boot is there, with what look like reasonable contents, but I'm no expert.

I then booted a live USB and ran 'boot-repair'. The output from its last re-install attempt is here:

http://paste.ubuntu.com/p/h6ybvcpKSV/

It fails with a similar error:

" Generating grub configuration file ... grub-probe: error: unknown filesystem.

An error occurred during the repair.

You can now reboot your computer. "

I am now at a loss as to where to go next - this isn't googling very well, I can't find anyone who actually found the root cause of an issue - only cases where a re-install or similar worked. I think I could install grub on a different partition, but that seems to be guesswork to me, rather than based on any evidence.

Any ideas anyone?

  • Update: I have booted the Super Grub2 Disk and got no further. It's very odd - it doesn't recognise anything on hd1,msdos1 (/dev/sda1) - simply stating 'No known filesystem detected'. I've triple-checked by booting Gpartd and checking & mounting /dev/sda1 again - it sees everything fine. So in essence it seems that grub can't recognise my / ext4 partition that both Gpartd and Live Ubuntu can mount no problem. – Digger Dec 10 '18 at 22:26
  • Update2: I've rebuilt the boot loader in the vain hope that it would help. Unlikely as grub-rescue knows that /dev/sda3 is 'ext2' (I assume that this is an artefact of the kernel module grub uses to read ext2/3/4). % grub-mkimage -O i386-pc -o /mnt/boot/grub/i386-pc/core.img -p '(hd0,msdos1)/boot/grub' biosdisk part_msdos ext2 % grub-bios-setup -d /mnt/boot/grub/i386-pc /dev/sda Anyway, same outcome. Still dumped into grub-rescue. – Digger Dec 11 '18 at 9:38
  • For the record, here's the (pretty standard) partition table: Device Boot Start End Sectors Size Id Type /dev/sda1 * 63 73865215 73865153 35.2G 83 Linux /dev/sda2 73865216 77008895 3143680 1.5G 82 Linux swap / Solaris /dev/sda3 77008896 488397167 411388272 196.2G 83 Linux – Digger Dec 11 '18 at 9:43
  • Update 3: I'm pretty sure it's this issue. bugzilla.redhat.com/show_bug.cgi?id=1529969 My FS has the inline_data flag set. Looks like it was introduced in grub 2.02 - which as far as I can tell is newly deployed in 18.04. Given that grub has 100s of unaddressed bugs savannah.gnu.org/bugs/… I doubt it'll be fixed any time soon. So separate boot partition time I think. – Digger Dec 11 '18 at 14:19
  • Update 4: I have a resolution, but no root cause. I followed help.ubuntu.com/community/BootPartition to create a new ext2 boot partition. – Digger Dec 11 '18 at 17:52

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