Wiki da Rede IME

Supondo que a particao de boot esteja em sda1

sudo mount /dev/sda1 /mnt
sudo mount -t proc none /mnt/proc
sudo mount -o bind /dev /mnt/dev
sudo mount -o bind /sys /mnt/sys
sudo mount -t devpts devpts /mnt/dev/pts
sudo chroot /mnt
sudo update-grub
sudo grub-install /dev/sda

http://linuxandfriends.com/how-to-reinstall-grub2-chroot-into-a-linux-partition/