How to Fix ‘Error: Unknown Filesystem Grub Rescue’

The dreaded “Error: Unknown filesystem grub rescue>” message can appear when your computer fails to boot. It indicates that the GRUB (Grand Unified Bootloader), responsible for loading your operating system, can’t find the partition it needs to boot. This usually occurs after a failed operating system update, partition resizing, or due to file system corruption. Fortunately, the issue can often be resolved using a few straightforward troubleshooting steps. This comprehensive guide will walk you through those fixes, allowing you to boot back into your OS.

Understanding the Problem

Before diving into the solutions, it’s crucial to grasp what’s causing this error. The GRUB bootloader resides in the Master Boot Record (MBR) or EFI partition and points to the location of your operating system’s kernel. The “Unknown filesystem” error signifies that GRUB cannot recognize the file system type of the partition where your kernel resides, rendering it unable to locate and load the OS. This can happen because:

File system corruption: The file system on the partition might be damaged, preventing GRUB from reading it correctly.
Incorrect GRUB configuration: The GRUB configuration file (grub.cfg) might contain incorrect information about the partition location or file system type.
Missing or damaged GRUB modules: Essential GRUB modules required to recognize the file system may be missing or corrupted.
Partition changes: Resizing, deleting, or moving partitions can invalidate GRUB’s configuration.
BIOS/UEFI issues: In some rare cases, BIOS/UEFI settings can interfere with the boot process.

Solutions to Fix the Error

Here are several methods to resolve the “Error: Unknown filesystem grub rescue>” issue. We will start with the simplest and most common solutions and then move to more advanced techniques.

1. Identifying the Boot Partition

The first step is to identify which partition contains your operating system. We’ll use GRUB’s built-in commands to do this.

List available partitions: At the grub rescue> prompt, type the following command and press Enter:
```
ls
```
This command will list all available partitions, typically named like (hd0,msdos1), (hd0,gpt1), (hd1,msdos2), etc. hd0 represents the first hard drive, hd1 the second, and so on. msdos or gpt indicates the partition table type (Master Boot Record or GUID Partition Table, respectively), and the number after it indicates the partition number.
Inspect each partition: Now, you need to inspect each partition to find the one containing your operating system. Try the following command for each partition, replacing (hd0,msdos1) with the actual partition name:
```
ls (hd0,msdos1)/ 
```
If the partition contains your operating system, you will see directories like boot, etc, usr, var, etc. Look for the boot directory, which is a strong indicator.

2. Setting the Prefix and Root Variables

Once you’ve identified the correct partition, you need to tell GRUB where to find its files and the operating system’s kernel.

Set the prefix variable: The prefix variable tells GRUB where to find its modules and configuration files. Use the following command, replacing (hd0,msdos1) with the correct partition:
```
set prefix=(hd0,msdos1)/boot/grub
```
Set the root variable: The root variable tells GRUB which partition contains the operating system’s root directory. Use the same partition as above:
```
set root=(hd0,msdos1)
```
Load the normal module: Now, load the normal module, which provides the familiar GRUB menu:
```
insmod normal
```
Run the normal command: Finally, execute the normal command:
```
normal
```
If successful, this should load the GRUB menu, allowing you to boot into your operating system. If this works temporarily, continue to the next step to make the changes permanent.

3. Regenerating the GRUB Configuration File

If you’ve successfully booted into your operating system using the above steps, you need to regenerate the GRUB configuration file to make the changes permanent. The process varies slightly depending on your operating system.

For Debian/Ubuntu-based systems:
Open a terminal and run the following command:
```
sudo update-grub
sudo grub-install /dev/sda
```
Replace /dev/sda with the correct disk where GRUB should be installed (usually the disk where your OS is installed). You can determine the correct disk using the lsblk command.
For Fedora/CentOS/RHEL-based systems:
Open a terminal and run the following command:
```
sudo grub2-mkconfig -o /boot/grub2/grub.cfg
sudo grub2-install /dev/sda
```
Again, replace /dev/sda with the correct disk. If you are using UEFI, it may be necessary to specify the EFI directory. Consult your distribution’s documentation for precise steps.

4. Using a Live CD/USB

If the previous methods fail, you might need to use a live CD or USB drive to repair GRUB. This involves booting from a live environment and then using specialized tools to reinstall GRUB.

Boot from a live environment: Download a live CD/USB image of your Linux distribution (or any other Linux distribution). Boot your computer from the live environment.
Mount your root partition: Identify your root partition using lsblk or fdisk -l. Then, mount the partition. For example:
```
sudo mount /dev/sda1 /mnt
```
Replace /dev/sda1 with the correct partition.

Mount necessary system directories: Mount the required system directories:

sudo mount --bind /dev /mnt/dev
sudo mount --bind /sys /mnt/sys
sudo mount --bind /proc /mnt/proc

Chroot into your system: Use the chroot command to change the root directory to your mounted partition:
```
sudo chroot /mnt
```
Reinstall GRUB: Now, you can reinstall GRUB using the appropriate command for your distribution (as described in the previous section).
```
sudo update-grub # Or grub2-mkconfig -o /boot/grub2/grub.cfg
sudo grub-install /dev/sda
```

Exit chroot and reboot: Exit the chroot environment and reboot your computer:

exit
sudo umount /mnt/dev
sudo umount /mnt/sys
sudo umount /mnt/proc
sudo umount /mnt
sudo reboot

5. Advanced Troubleshooting

If none of the above methods work, the problem might be more complex. Some advanced troubleshooting steps include:

Checking the BIOS/UEFI settings: Ensure that your BIOS/UEFI settings are configured correctly. Specifically, check the boot order and make sure your hard drive is listed as a bootable device. Secure Boot may also interfere, so try disabling it temporarily.
Testing your hard drive: Use a diagnostic tool to check your hard drive for errors. Bad sectors or other hardware issues can prevent GRUB from reading the partition correctly.
Examining the GRUB configuration file: Manually inspect the grub.cfg file for errors. This file is usually located in /boot/grub or /boot/grub2. Look for incorrect partition UUIDs or other configuration problems.

Conclusion

The “Error: Unknown filesystem grub rescue>” can be intimidating, but with careful troubleshooting, you can often resolve it. By identifying the correct boot partition, setting the prefix and root variables, regenerating the GRUB configuration file, or using a live CD/USB, you can restore your system to a bootable state. Remember to back up your important data before attempting any of these fixes, especially if you suspect hardware issues. Good luck!

How to Fix ‘Error: Unknown Filesystem Grub Rescue’#

Understanding the Problem#

Solutions to Fix the Error#

1. Identifying the Boot Partition#

2. Setting the Prefix and Root Variables#

3. Regenerating the GRUB Configuration File#

4. Using a Live CD/USB#

5. Advanced Troubleshooting#

Conclusion#