Ez a sort így alkalmaztad a leírásból?
grub-install --recheck /dev/sda
Kénytelen vagyok idézni (ez egy kicsit off) :)
Imagine for any reason you have any other crappy OS installed next to .... And for any reason you need to reinstall it because is really broken. Again, imagine this crappy OS has a crappy installation and overwrites the disk MBR to own the full computer. What's next? Reinstall? No.