sudo mount /dev/sda5 /mnt
sudo grub-install --root-directory=/mnt/ /dev/sda

grub-probe: error: cannot find a device for /boot (is /dev mounted?).
Installation finished. No error reported.

root (hd0,0) (where is /boot with /boot/grub/grub.conf and grub stages files ?)
setup (hd0) (where do you want to install the bootloader?)
exit