Problems with Target Display Mode

Use your iMac as a display with target display mode - Apple Support

If target display mode doesn't work

If your iMac doesn't show the desktop of your other Mac, try these steps first:

1. Restart your iMac.
2. Restart the other Mac.
3. Unplug the Thunderbolt or Mini DisplayPort cable from the other Mac, then plug it back in.
4. Press Command-F2 on the iMac.

If that doesn't work, try these other solutions:

• If you're currently logged in to the iMac that you want to use as a display, choose Apple menu  > Log Out to return to the login window. Then press Command-F2 again.
• Choose Apple menu  > System Preferences, then click Keyboard. If ”Use F1, F2, etc. keys as standard function keys” is selected, target display mode uses Command-Fn-F2 instead of Command-F2. It might also help to use the keyboard that came with your iMac. Some third-party keyboards and older Apple keyboards don't support target display mode. 
• Make sure that your iMac is using macOS High Sierra 10.13.6 or earlier. You can't use target display mode with later versions of macOS, or with Boot Camp and Windows.

I find I have best results when the Target Display Mode computer is at the login screen. I then restart the master/main computer and many times it will connect and work.

If it doesn’t work, I then unplug the cable from the back of the master/main computer for a few seconds, plug back in and the TDM computer/screen will switch over in 5 to 10 seconds.

Ive also had a lot of issues with 3rd party keyboards not activating TDM when F2 is pressed.

Not sure any of this will work, but nothing else seems have either.

tin225 wrote:

I also booted both devices in Safe Mode, which made both of these devices' screens look absolutely crippled.

Safe Mode disables accelerated graphics.

You need to wait until someone can help you.

You're sure the iMac is a 2011 model? Have you tried a different thunderbolt cable? Different keyboard?

I was wondering that also.

Thanks for the tips Jeff. :)