How to reinstall GRUB bootloader

grub_re_install

Those that have Legacy instead of EFI can go to step 4)
For EFI there is one extra step: Mount your EFI partition

    mount /dev/sda1 /mnt/boot/efi

Sometimes errors like this can result: mount: /mnt/boot/efi: mount point does not exist.
To solve it create the efi directory in /mnt/boot/ using this command:

    mkdir /mnt/boot/efi

then try to mount the efi partition again.

    Now we have to chroot in the system (i let you google what chroot means :wink:)

    chroot /mnt

Now that you are in you can just install the grub by typing this command: (grub-install ask for disk, not a partition so dont put any number like this: sda instead of sda1

    grub-install /dev/sda

then if no errors are chown you can just exit by typing:

    exit

and reboot the system. Once the system is rebooted, run update-grub to be sure that any other OS is detected by the grub.

    sudo update-grub

If you have any issue you can always ask here.

https://community.parrotsec.org/t/tutorial-installing-grub-using-live-usb/5672