Target Disk Mode after High Sierra Installs

COMMAND-F2, I believe, is used to switch Target Display Mode. To end Target Disk Mode just requires ejecting the drive from the other machine then shutting down or restarting the source machine. See Target Disk Mode.

I have the same issue exiting from the target display mode since High Sierra. For now, I'm rebooting the iMac remotely (ssh) with the command: sudo reboot

Use your iMac as a display with Target Display Mode - Apple Support

If Command-F2 doesn't work

If your iMac doesn't switch to Target Display Mode when you press Command-F2, check these things:

• Make sure that the iMac you're trying to use as a display supports Target Display Mode.
• Target Display Mode works only when your primary Mac is logged in to a macOS user account. For example, if your first Mac is at the FileVault login screen, pressing Command-F2 on your iMac does not enable Target Display Mode. You must first log in to your Mac (using a regular display, if necessary) before you can use an iMac in Target Display Mode.
• Make sure that you're pressing Command-F2 on the keyboard that's connected to the iMac you want to use as a display.
• In Keyboard System Preferences, if the checkbox is enabled for "Use all F1, F2, etc. keys as standard function keys," the key combination changes to Command-Fn-F2.
• Make sure that your Mini DisplayPort or Thunderbolt cable is connected to both computers.
• If you're currently logged in on the iMac you want to use as a display, try logging out to return to the login window. Then press Command-F2 again.
• Some older Apple keyboards and third-party keyboards might not allow Command-F2 to toggle display modes. If this happens, use the keyboard that came with your iMac to toggle Target Display Mode on and off.
• If your iMac is currently started up in Windows, it won't enter Target Display Mode. Target Display Mode isn't supported in Boot Camp.

I found the following in the logs exiting target display mode:

Jan 10 22:25:38 --- last message repeated 2 times ---

Jan 10 22:25:38 iMac com.apple.xpc.launchd[1] (com.apple.dpd[269]): Service exited with abnormal code: 75

Jan 10 22:25:46 iMac com.apple.xpc.launchd[1] (com.apple.preference.displays.MirrorDisplays): Service only ran for 0 seconds. Pushing respawn out by 10 seconds.

Jan 10 22:25:48 iMac com.apple.xpc.launchd[1] (com.apple.dpd[1208]): Service exited with abnormal code: 75

Jan 10 22:25:56 iMac com.apple.xpc.launchd[1] (com.apple.preference.displays.MirrorDisplays): Service only ran for 0 seconds. Pushing respawn out by 10 seconds.

Jan 10 22:25:59 iMac com.apple.xpc.launchd[1] (com.apple.dpd[1210]): Service exited with abnormal code: 75

The problem JeffBox outlined is EXITING target display mode. Whether turning off the connected Macbook, or exiting target display mode using Command-F2, the iMac hangs with a completely black screen and must be restarted with the power button. This is clearly a bug in High Sierra 10.13.1. Please sort it out Apple, because a lot of Macbook users rely on this functionality.

I will ask if the topic and other changes can be edited by the moderators.

You are right, I have the same problem while exiting target display mode.

My OS is 10.13.2, dry me crazy