Fix GRUB Rescue Unknown Filesystem on Ubuntu or Debian

Before you begin

Match the environmentUse this guide only for Ubuntu, Debian or a clearly compatible setup.

Protect recoveryBack up important data and save encryption or account recovery keys before changing system state.

Know when to stopThis guide includes advanced changes. If the device contains irreplaceable data or belongs to an employer, stop and use a qualified technician.

Problem overview

The computer stops at error: unknown filesystem and a grub rescue> prompt. GRUB cannot find the partition or module directory it was configured to use.

Caution

Do not copy /dev/sda or an example partition name into a write command. SATA, NVMe, USB, RAID, and multi-drive systems use different identifiers. Writing GRUB to the wrong disk can make another operating system unbootable.

Step 1: Check firmware boot order

If the problem started after adding a drive or changing firmware, open the UEFI/BIOS boot menu and select the existing Ubuntu or Debian boot entry. If that works, restore the intended entry to the top of the boot order and do not reinstall GRUB.

Step 2: Find the Linux partition from GRUB rescue

List detected disks and partitions:
```
ls
```
Inspect candidates one at a time, using names from that output:
```
ls (hd0,gpt1)/
ls (hd0,gpt2)/
```
Look for a partition containing /boot/grub, or a root filesystem containing directories such as /etc, /home, and /var.
Set the discovered partition. Replace the example below with the partition that actually contained /boot/grub:
```
set root=(hd0,gpt2)
set prefix=(hd0,gpt2)/boot/grub
insmod normal
normal
```

If insmod normal still reports an unknown filesystem, stop guessing and continue with a live USB.

Step 3: Inspect the installation from a live USB

Boot an Ubuntu or Debian live USB in the same firmware mode as the installed system.

Open a terminal and collect read-only information:

lsblk -f
test -d /sys/firmware/efi && echo UEFI || echo Legacy-BIOS

Identify the Linux root filesystem and, on UEFI systems, the small FAT32 EFI System Partition. Do not format either partition.
Mount the root filesystem, replacing the example device with the identifier shown by lsblk -f:
```
sudo mount /dev/nvme0n1p2 /mnt
```
On UEFI systems, mount the existing EFI System Partition at the location used by the installation, commonly /mnt/boot/efi.

Step 4: Reinstall with distribution-aware help

GRUB reinstall commands differ between UEFI and Legacy BIOS and depend on whether /boot is separate. Use the current Ubuntu or Debian boot-repair documentation for the detected mode, or ask a qualified Linux administrator to confirm the mount layout before running grub-install.

After reinstalling, run update-grub, restart without the live USB, and confirm both Linux and any other operating-system boot entries still work. If they do not, return to the firmware boot menu rather than repeatedly reinstalling to different disks.

Sources

Confirm the result

Repeat the exact action that originally triggered the problem.

Confirm the original error or symptom is gone and no new warning has appeared.

If the guide changed a driver, service, package, or system setting, restart once and test again.

If the result is worse or unexpected, stop. Reverse only the last change using its documented restore option; if none is documented, use your backup or qualified support.

Questions about this fix

Why did GRUB suddenly report an unknown filesystem?

Common causes include moving or deleting a partition, a changed filesystem identifier, an interrupted bootloader update, or firmware boot-order changes.

Does the rescue prompt mean my files are gone?

Not necessarily. GRUB may have lost the path to its modules even when the Linux filesystem is intact. Confirm the partition before writing to any disk.