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My external monitor works fine but only after I log in using the MacBook display. Prior to logging in it sends no signal. Once logged in, it suddenly starts receiving signal and works exactly as expected.

The cable connecting it to my display is a USB-C to HDMI.

How can I make it work correctly before logging in?

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  • Prior to logging in, there’s nothing to see. When it boots, all console messages are sent to the primary display so your screen will stay asleep. You login, likewise, will also only be on the primary display. What are you hoping/expecting to see on the secondary display prior to login?
    – Allan
    Commented Aug 21, 2020 at 0:07
  • 3
    I have the displays mirrored so I was hoping to see the logon screen. The MacBook sits at the back of my desk partially hidden by the external monitor and I use the external monitor as my main screen so it’s a little inconvenient that I have to move it to see the screen better just to log in.
    – james hobday
    Commented Aug 21, 2020 at 7:35
  • Mirroring is a function of the OS not the computer itself, so it can’t mirror before it loads. If you want to have that screen become your main screen during boot, either close the lid or put a magnet near the center along the front edge do it triggers the sensor that detects the lid being closed.
    – Allan
    Commented Aug 21, 2020 at 15:39

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How can I make it work correctly before logging in?

It is working correctly. Your built in screen is the main screen and during boot all boot/console messages are sent (they’re usually hidden by the Apple logo). The login screen is set to display on the main screen so until you login to the GUI, there’s nothing to display on the secondary display.

But, based on your comments, what you want to do is use your Mac in closed lid or closed clamshell mode:

I have the displays mirrored so I was hoping to see the logon screen. The MacBook sits at the back of my desk partially hidden by the external monitor and I use the external monitor as my main screen so it’s a little inconvenient that I have to move it to see the screen better just to log in

If the display is partially hidden/obstructed and you have the contents mirrored and not as an extended display, this is a perfect scenario to use the Mac in closed clamshell mode. The secondary/external display will become the main display and boot and login screens will appear there.

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    "The secondary/external display will become the main display and boot and login screens will appear there." not for me on my MPB
    – tir38
    Commented Feb 13, 2021 at 22:59

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