How to Reinstall GRUB in Ubuntu Using a Live USB to Fix Boot Errors

Why Did GRUB Break?

GRUB (GRand Unified Bootloader) is the crucial piece of software that tells your computer how to load the Ubuntu operating system. If you attempt to dual-boot Windows and Ubuntu, a major Windows update will frequently overwrite the Master Boot Record (MBR) or the EFI partition, completely erasing GRUB. When you turn on your PC, instead of seeing the familiar OS selection menu, you are met with a terrifying black screen displaying a “No bootable device found” or “grub rescue>” error. Your Ubuntu files are completely safe; the system simply forgot how to find them. To fix this, you must reinstall GRUB using a live Ubuntu USB drive.

Step 1: Boot into a Live Environment

You cannot fix the bootloader from a broken operating system. You need to boot from external media.

1. Insert a bootable Ubuntu Live USB drive (the same kind you used to install Ubuntu initially).
2. Restart your computer and rapidly press your motherboard’s boot menu key (usually F12, F10, F2, or Del).
3. Select the USB drive to boot from.
4. When the Ubuntu installation screen appears, choose Try Ubuntu without installing. This loads a temporary desktop environment directly from the USB.

Step 2: Connect to the Internet and Install Boot-Repair

While you can repair GRUB manually using a complex series of chroot terminal commands, the easiest and safest method is to use a graphical tool called Boot-Repair.

1. Once you are on the temporary Ubuntu desktop, connect to your Wi-Fi or plug in an Ethernet cable.
2. Open the Terminal (Ctrl + Alt + T).
3. You need to add the repository for the Boot-Repair tool. Run this command:
4. sudo add-apt-repository ppa:yannubuntu/boot-repair
5. Press Enter to confirm.
6. Update your package list:
7. sudo apt update
8. Install the tool:
9. sudo apt install -y boot-repair

Step 3: Run the Recommended Repair

Once installed, the graphical tool will do all the heavy lifting.

1. In the Terminal, launch the tool by typing:
2. boot-repair
3. Press Enter. The application window will open and perform a quick scan of your hard drives.
4. Click the large button labeled Recommended repair (repairs most frequent problems).
5. The tool will begin reinstalling GRUB to the correct partition. It will automatically detect your Windows installation and add it back to the boot menu as well.
6. When it finishes, it will display a success message and generate a URL with a log report (you only need the URL if the repair failed).
7. Reboot your computer and physically remove the USB drive. The GRUB menu will reappear, allowing you to boot normally.