My laptop recognises external display devices if I connect them before booting the system, but many times it doesn't if I connect while the system is running. In this case, rebooting naturally solves the problem.
Is there a way to force Ubuntu to detect external displays? Opening up the display menu and pressing "Detect displays" does nothing.
Actually, you don't need to log out. Simply going to a VC with ctrl-alt-F1, restarting x with sudo service sddm restart and
and going back to your graphical interface with ctrl-alt-F7 (or F2) should do it.
This way, you don't lose all your windows...
To enable all outputs in their default mode, run:
$ xrandr --auto
For more information, see:
https://xorg-team.pages.debian.net/xorg/howto/use-xrandr.html
I just encountered this issue with my Dell Latitude E5550 using a port extender and two external monitors.
My problems began when I had a power failure. For silly reasons, I could only plug in one of my two monitors, so I opened up the screen display interface and de-activated the screen that didn't have power.
Awesome!
Once the power came back on the process to turn my screen back on was unintuitive. I couldn't see it in the screen display interface even after rebooting or redocking my laptop.
So I tried $ xrandr --auto and got Ubuntu to extend to "one" screen but duplicate to both of my externals (they are two of the same model). Amusing but not useful.
It was only when I clicked on "1 Built-in display" like in the screenshot below that I was able to select my "3rd" screen and turn it on again.
Doing this flipped the two screens around for some reason and I had to rearrange them but that was easily fixed!
Adapting Amanda's solution to modern ubuntu with systemd and/or other display manager than sddm :
"Actually, you don't need to log out. Simply going to a VC with ctrl-alt-F1, restarting x with sudo systemctl restart <your display manager> (eg gdm or lightdm) and going back to your graphical interface with ctrl-alt-F7 (or F2) should do it.
This way, you don't lose all your windows..."
I also have Dell Latitude E5550, I have one onitor via hdmi, and one via usb-c. Then one via usb-c is not detected sometimes, and for me what it works, is unplug/plug the hdmi port in the monitor, while the rest is connected.
Working solution: Disable secure boot in the bios/efi menu.
Hello Everyone, I am coming from a TongFang GK5CN6Z and I had this issue. I noticed it was an issue after having ran the latest updates on Ubuntu. Turns out I had to disable secure boot in order for it to work.
I had issues with dual monitors when using prime-select intel when proprietary Nvidia drivers are installed. I guess either uninstalling Nvidia drivers or using only prime-select nvidia works, though Nvidia drivers have their own problems.
If the external monitor can't be detected by using xrandr --auto, or even by booting to different OS, reboot Ubuntu using a previous kernel. If the computer detects the monitor in the older kernel, you can go back to the latest one without any problems.
I don't know the exact reason why it works, but it definitely worked for me.
