Dave writes, “I have an old MacBook laptop with a broken monitor (it just displays vertical lines). I connected it to my HDTV using an HDMI cable and a DVI to HDMI adapter and all I got on my TV was a blue screen. I had the TV set to the right input. Any ideas about how to solve my Mac external display problem?”
Connecting an external display to a MacBook, as you did when you connected your HDTV, typically causes the Mac to extend the desktop to the external display. Sometimes this doesn’t happen automatically, which means you need to make manual changes. In your case, the Mac shouldn’t try to use the built-in laptop screen because it’s broken, but the Mac probably can’t detect that. Fortunately there’s a relatively simply solution to the external display problem you are having.
By default, macOS assumes you want to extend your desktop to the external monitor, as opposed to mirroring what would normally show up on the built-in screen. You need one extra piece of hardware to get around this – an external keyboard.
Overriding Mirroring on an External Display
Since you can’t see anything on the broken screen, you obviously don’t need to use it.
Instead, what you want to do is connect the external monitor or HDTV as you already did. Then connect an external USB keyboard and close the Mac. This tells Mac OS X that you are using external hardware and won’t cause the Mac to go to sleep when you close it.
If you already paired a Bluetooth keyboard with your Mac, that will also work, but you won’t be able to see the configuration screen if you haven’t paired it. Keep in mind, you will also want a mouse or trackpad, since you won’t have access to the trackpad on your MacBook while the screen is closed over the keys.
Mac External Display Resolution Problems
Sometimes connecting an external display results in issues where the resolution looks wrong. This typically happens when macOS attempts to use a resolution that’s incompatible with the display causing the screen to appear stretched. Or sometimes it will pick a lower resolution than maximum.
If either of these events happen, open System Settings and locate Displays. From there choose one of the supported resolutions. I typically choose the maximum supported, which often is also the “default” resolution in the list.
Internal Graphics Card Can’t Power External Display Correctly
Another issue that comes up on occasion is the external display problems stem from having a monitor that has a higher resolution than what is supported by your Mac’s graphics card. When that happens, choosing an external display adapter can help address the issue. This is most common when you are trying to connect two monitors to a laptop.
