grub rescue> error: unknown filesystem
I recently merged two partitions on my hard drive using Disk Management system utility in Windows 10.
Since I use the same drive to dual-boot Windows 10 and Arch Linux, having one less partition on my drive changed the GPT partition numbers and GRUB was unable to recognize the right partition where to locate its modules.
I encountered this same problem several times in the past, so I just googled the error message trying to remember the few commands that I need to run to temporarily set the right partition in GRUB and be able to boot normally.
This post is mostly a reminder for myself since I always forget these steps. (and for you, if your Google search led you here)
Now, let’s dive into the actual solution for this error.
The error message #
This is how the error message looked like exactly.
error: unknown filesystem. Entering rescue mode... grub rescue>
The solution I usually use #
This error frequently happens to GNU/Linux users. Therefore, there are plenty of posts and videos explaining how to fix it.
My Google search always lead me to this Ask Ubuntu answer.
In addition, having a look at the ArchWiki is always a good idea. I recommend reading this wiki page about GRUB to get a better understanding of what’s really happening with your GNU/Linux system.
Guessing the boot partition number #
If you don’t memorize your partition table by heart like me, your first mission
is to guess the partition where your
directory is located.
There are three common scenarios.
- Your boot partition is encrypted: If you fall into this category then you’ve chosen the wrong guide. However, this blog post may be helpful for you.
/boot/directory is located in the root partition: You’ll have to find which partition is your root partition. It is usually recommended to use
ext4filesystem for root partitions. A typical GNU/Linux directory structure is explained in this post.
- You have a separate boot partition: I installed my Arch Linux using LVM
system encryption method. Hence, I have a separate boot partition that I
ext4are all supported for the boot partition on modern GNU/Linux distributions.
Listing all partitions #
The first step is to list all partitions.
grub rescue> ls (hd0) (hd0,gpt6) (hd0,gpt5) (hd0,gpt4) (hd0,gpt3) (hd0,gpt2) (hd0,gpt1)
Filesystem of each partition #
For each partition, we need to find out what filesystem is being used.
grub rescue> ls (hd0,gpt1) (hd0,gpt1): Filesystem is unknown.
If the filesystem is unknown then we just ignore the partition.
I tried this same command with all partitions until I found out that my
partition is using
ext2. I don’t have any
ext2 partition, but I guess that
GRUB is not very accurate and mistakes
grub rescue> ls (hd0,gpt5) (hd0,gpt5): Filesystem is ext2.
The content of ext2 partitions #
At this stage, we need to find out where GRUB modules are located.
/ to the last command to show the content of
grub rescue> ls (hd0,gpt5)/ ./ ../ lost+found/ vmlinuz-linux initramfs-linux.img initramfs-linux-fallback.img grub/
The GRUB modules directory #
grub/ is the directory we’re trying to find.
Let’s list the content of
grub/ to make sure it’s the right GRUB modules directory.
grub rescue> ls (hd0,gpt5)/grub/ ./ ../ x86_64-efi/ themes/ fonts/ grubenv grub.cfg
/boot/ directory is located in the root partition, your GRUB modules
path is probably
We have to set
Make sure to replace
gpt5 by the partition where your
/boot/ directory is
grub rescue> set root=(hd0,gpt5)
prefix path depends on where your GRUB modules directory is located. Make sure
to read the previous section of this post to find out the right path.
grub rescue> set prefix=(hd0,gpt5)/grub
We load the
grub rescue> insmod normal
And finally, we try booting the system normally.
grub rescue> normal
Fix boot partition permanently #
In UEFI systems, the boot partition is
hard-coded in the
This file is usually located in
/esp/EFI/grub/ in Arch Linux.
I don’t know if there is a way to safely edit the hard-coded value in this file, so I just reinstalled my GRUB.
Find out your EFI system partition mount point #
grub-install does three things:
- Generates a new
grubx64.efifile in your EFI system partition;
- Adds a new boot option for the generated
- Installs GRUB modules to
Therefore, your EFI system partition needs to be mounted.
On my Arch Linux, my EFI system partition gets automatically mounted to
fstab. This may be the
case on your GNU/Linux system too.
Let’s detect the EFI system partition using
$ sudo fdisk -l /dev/sda | grep -i efi /dev/sda2 1085440 1290239 204800 100M EFI System
sda2 is my EFI system partition.
Then, we use
df to determine the mount point of
$ df | grep sda2 /dev/sda2 98304 32830 65474 34% /esp
If your EFI system partition does not get automatically mounted at startup, you
need to mount it manually. We will use
/esp/ as the mount point (use a
different path if
/esp/ directory already exists on your system).
$ sudo mkdir /esp && sudo mount /dev/sda2 /esp
Reinstalling GRUB #
Reinstalling GRUB depends highly on how you initially installed your GNU/Linux distribution. I use Arch Linux installed in UEFI mode and the following command did the trick for me.
$ sudo grub-install --target=x86_64-efi --efi-directory=/esp/ --bootloader-id=GRUB