buntu@ubuntu:~$ sudo mount /dev/sdb3 /mnt
ubuntu@ubuntu:~$ sudo grub-install --root-directory=/mnt/ /dev/sdb3
/usr/sbin/grub-setup: warn: Attempting to install GRUB to a partition instead of the MBR.  This is a BAD idea..
/usr/sbin/grub-setup: warn: Embedding is not possible.  GRUB can only be installed in this setup by using blocklists.  However, blocklists are UNRELIABLE and their use is discouraged..
/usr/sbin/grub-setup: error: if you really want blocklists, use --force.
ubuntu@ubuntu:~$