GRUB Errors in Ubuntu: Causes and Bootloader Fix Methods

What a GRUB Error Means

GRUB errors in Ubuntu manifest in several forms:

• grub> screen — a minimal GRUB command-line interface that appears when it cannot find the grub.cfg configuration file.
• grub rescue> screen — an even more limited mode, activated when the core bootloader or partition is damaged.
• error: no such partition message — GRUB cannot find the partition with the installed system.
• Ubuntu missing from the menu — the system does not appear in the boot list, even though it is installed.
• Hanging on a black screen after selecting an item — likely a corrupted configuration or kernel.

These symptoms mean the bootloader cannot properly read the configuration or hand over control to the Linux kernel. Most often, the problem occurs after:

• Interrupting a system or kernel update.
• Manually changed GRUB settings (e.g., in /etc/default/grub).
• Partition damage (disk failure, incorrect partition table modification).
• Reinstalling Windows, which overwrote the MBR/ESP.
• Errors during disk cloning or moving the system to a different drive.

Common Causes

1. Corrupted grub.cfg file — the configuration file may be deleted, damaged, or not updated after a new kernel installation.
2. Incorrect settings in /etc/default/grub — errors in parameters (e.g., GRUB_CMDLINE_LINUX or GRUB_TIMEOUT) can make GRUB non-functional.
3. Windows bootloader overwrite — during Windows installation or recovery, its bootloader overwrites the MBR (in BIOS) or the EFI partition entry (in UEFI), removing GRUB.
4. Partition table changes — moving, deleting, or changing the types of partitions where GRUB resides (especially /boot or the EFI system partition).
5. Insufficient space in /boot — when the /boot partition fills up, kernel updates may not install completely, breaking the boot process.
6. GRUB module errors — damage to files in /boot/grub (e.g., core.img) due to disk failure or interrupted writes.
7. Boot mode mismatch (BIOS/UEFI) — if Ubuntu was installed in UEFI mode but the BIOS is set to Legacy, or vice versa, GRUB will not be found.

Solutions

Method 1: Restore GRUB with Boot-Repair (Graphical Method)

Boot-Repair is a utility that automatically diagnoses and fixes most GRUB problems. Suitable for beginners and quick resolution.

1. Boot from an Ubuntu Live USB/DVD (as described in the howToSteps above).
2. Connect to the internet (via Wi-Fi or wired).
3. Open a terminal and run:

   sudo add-apt-repository ppa:yannubuntu/boot-repair
   sudo apt update
   sudo apt install boot-repair
   boot-repair
   

4. In the window that opens, click "Recommended repair".
5. Wait for completion (may take 5-10 minutes). The utility will automatically reinstall GRUB, update the configuration, and fix common errors.
6. After completion, close the window, reboot, and remove the Live USB/DVD.

⚠️ Important: If Boot-Repair offers to create a report (paste URL), save it — it will help with further diagnostics if the problem persists.

Method 2: Manual GRUB Restoration via chroot (Command Line)

This method gives full control and works when Boot-Repair fails or precise configuration is needed.

1. Boot from an Ubuntu Live USB/DVD and open a terminal.
2. Identify the Ubuntu partition (see howToSteps, step 2). Assume it is /dev/sda3.
3. Mount the root partition:

   sudo mount /dev/sda3 /mnt
   

   If you have a separate /boot partition, mount it:

   sudo mount /dev/sdaX /mnt/boot  # replace X with your /boot partition number
   

   If using UEFI, mount the EFI system partition (usually FAT32, 100-500 MB):

   sudo mount /dev/sdaY /mnt/boot/efi  # replace Y with your EFI partition number
   

4. Mount virtual filesystems:

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

5. If using chroot with IPv6 addressing, you may also mount /run:

   sudo mount --bind /run /mnt/run
   

6. Perform chroot:

   sudo chroot /mnt
   

   You are now in your installed Ubuntu.
7. For BIOS (Legacy) systems:

   grub-install /dev/sda
   update-grub
   

   For UEFI systems:

   grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=ubuntu --recheck
   update-grub
   

   If the grub-install command fails, check if the EFI partition is mounted and contains the EFI/ubuntu folder.
8. Exit chroot: exit.
9. Unmount all partitions:

   sudo umount -R /mnt
   

10. Reboot.

Method 3: Reinstall GRUB (If Previous Methods Fail)

Sometimes a complete GRUB reinstall with cleanup of old data helps.

1. Complete steps 1-6 from Method 2 (chroot).
2. Remove old GRUB files:

   rm -rf /boot/grub
   rm -rf /boot/efi/EFI/ubuntu  # UEFI only
   

3. Reinstall GRUB as in step 7 of Method 2.
4. Update the configuration:

   update-grub
   

5. Exit, unmount, and reboot.

Method 4: Fix GRUB Configuration (If the Error is in the Config)

If GRUB loads but cannot find the kernel or shows errors like error: file '/boot/vmlinuz-...' not found, the issue may be in the configuration file.

1. Boot into Ubuntu (via Rescue mode or if booting partially works).
2. Open the /etc/default/grub file:

   sudo nano /etc/default/grub
   

   Check parameters:
   • GRUB_CMDLINE_LINUX — should not contain invalid options.
   • GRUB_TIMEOUT — a reasonable value (e.g., 5-10 seconds).
   • GRUB_DISTRIBUTOR and GRUB_OS_PROBER — usually left at defaults.
3. If you recently edited this file, try temporarily commenting out changes (add # at the start of the line).
4. Update the configuration:

   sudo update-grub
   

5. Reboot.

If the system still does not boot, the kernel files may be the issue. Check for files in /boot:

ls -la /boot

Ensure files like vmlinuz-<kernel-version> and initrd.img-<kernel-version> exist. If not, reinstall the kernel:

sudo apt install --reinstall linux-image-generic
sudo update-grub

Prevention

To avoid recurring GRUB errors:

1. Do not interrupt system updates — especially processes involving the kernel (linux-image-*) or GRUB (grub-pc, grub-efi). Use sudo apt update && sudo apt upgrade in a stable state.
2. Back up critical partitions (e.g., /boot and the EFI partition) before major changes. A simple way:

   sudo cp -a /boot /boot-backup
   sudo cp -a /boot/efi /boot-efi-backup  # for UEFI
   

3. Avoid manually editing files in /boot/grub — changes should be made via /etc/default/grub and update-grub.
4. Maintain consistent boot mode — do not switch between BIOS and UEFI without reinstalling the system. Check the current mode:

   [ -d /sys/firmware/efi ] && echo UEFI || echo BIOS
   

5. Monitor free space on /boot — when it fills up (over 90%), remove old kernels:

   sudo apt autoremove --purge
   

   Or manually:

   dpkg -l 'linux-image*' | grep '^ii'  # list installed
   sudo apt remove linux-image-<old-version>
   

6. When installing Windows alongside Ubuntu — always boot from a Live USB and restore GRUB after completing the Windows installation.

Following these recommendations significantly reduces the risk of bootloader errors. If a problem occurs, start with Boot-Repair — it solves up to 80% of cases. In complex situations, use manual recovery via chroot.