By modifying grub, you can force the boot screen to a particular display. However, I've not found a way to have the screen displayed on both displays, which leads to the following warning:
Caveat: If the boot screen is directed to an external monitor, and it is not available, then you will not be able to see the boot screen and must boot blind [e.g. pressing return and entering password] to run Linux.
Open Terminal (CtrlAltT in Ubuntu).
xrandr to determine display devices. Note which display device is currently in use, and which you wish to use during booting.
- LVDS-1 (Low-Voltage Differential Signalling), may be used for a laptop LCD display.
- eDP-1 is also used for a laptop LCD display.
- HDMI-1 is the HDMI output.
- VGA-1 is the VGA output.
Enter (or paste, CtrlShiftV) [use nano if you prefer it to gedit]:
sudo gedit /etc/default/grub
Modify the line containing
GRUB_CMDLINE_LINUX to show the preferred boot device.
If it is currently commented out by a leading hash sign,
#, remove that symbol.
Add devices you wish to use, using
e for enable,
d for disable, and
D for digital output. For example, the following should enable both HDMI and LVDS, if they are listed as
LVDS-1 in the results of the
This may need some tweaking, e.g. disabling one device or another.