sudo mkdir /media/sda6
sudo mount /dev/sda6 /media/sda6
sudo mount -o bind /dev /media/sda6/dev/
sudo mount -o bind /sys /media/sda6/sys
sudo mount -t proc none /media/sda6/proc/
sudo chroot /media/sda6/
sudo update-grub
sudo grub-install /dev/sda