And the other option is to chroot and update/install grub from there (not mine, just copied from another list) :

mkdir /sysroot
mount /dev/your-root-dev /sysroot
mount /dev/your-boot-dev /sysroot/boot
mount --bind /dev /sysroot/dev
mount --bind /sys /sysroot/sys
mount --bind /proc /sysroot/proc
mount --bind /run /sysroot/run (recommended if you are using systemd)
chroot /sysroot
grub-install /dev/your-grub-boot-device (may be grub2-install on some distro)


This is geared to installing on another disk, but should work just the same with multiple OSs on one disk.
(/run left in for completeness)