it@it:~$ sudo 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
  --root-directory=DIR    install GRUB images under the directory DIR
                          instead of the root directory
  --grub-shell=FILE       use FILE as the grub shell
  --no-floppy             do not probe any floppy drive
  --force-lba             force GRUB to use LBA mode even for a buggy
                          BIOS
  --recheck               probe a device map even if it already exists

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

grub-install copies GRUB images into the DIR/boot directory specfied by
--root-directory, and uses the grub shell to install grub into the boot
sector.

Report bugs to <bug-grub@gnu.org>.
it@it:~$