Forcing grub menu to external monitor connected to laptop via HDMI/VGA

Here is a bad way to do this, for most, that may just be good for you.

On desktop, open this file as root

gksu gedit  /etc/default/grub

and find this line

GRUB_CMDLINE_LINUX_DEFAULT=...

and add a this to the end

video=LVDS-1:d

my whole line is

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

so I would add it here

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash video=LVDS-1:d"

Change lvds-1 for you monitors name, if it is different, find it with

xrandr -q

Save and exit, and run the command

sudo update-grub

This will disable the Laptop display for both grub and the desktop and force it to the external display, even if you remove the HDMI cable, the laptop screen will not work. You can remove the change to change it back to using the laptop screen.

I can't test this as I don't have a laptop, but I took it from an question at arch, where the issue was no laptop display after forcing grub to an external monitor.

I don't recommend this for most users who want there laptop to still be portable.