root@ubuntu:/tmp# grub-install --boot-directory=/mnt/boot/ /dev/sda
Unrecognized option `--boot-directory=/mnt/boot/'
Usage: grub-install [OPTION] install_device
Install GRUB on your drive.

  -h, --help              print this message and exit
  -v, --version           print the version information and exit
  --modules=MODULES       pre-load specified modules MODULES
  --root-directory=DIR    install GRUB images under the directory DIR
                          instead of the root directory
  --grub-setup=FILE       use FILE as grub-setup
  --grub-mkimage=FILE     use FILE as grub-mkimage
  --grub-probe=FILE       use FILE as grub-probe
  --no-floppy             do not probe any floppy drive
  --recheck               probe a device map even if it already exists
  --force                 install even if problems are detected
  --disk-module=MODULE    disk module to use

INSTALL_DEVICE can be a GRUB device name or a system device filename.

grub-install copies GRUB images into /boot/grub (or /grub on NetBSD and
OpenBSD), and uses grub-setup to install grub into the boot sector.

If the --root-directory option is used, then grub-install will copy
images into the operating system installation rooted at that directory.

Report bugs to <bug-grub@gnu.org>.