Apple에서 권장하는 모든 단계 를 시도 했습니까 ?

대상 디스플레이 모드 사용

다음 단계에 따라 iMac을 모니터로 사용하십시오.

1   Make sure that your iMac is turned on, and the other Mac is logged in to a macOS user account.
2   Connect the Mini DisplayPort or Thunderbolt cable between the two computers.
3   Press Command-F2 on the keyboard of the iMac that you want to use as a display.

Command-F2를 누르면 iMac에 연결된 Mac의 데스크탑이 표시됩니다.

Command-F2가 작동하지 않는 경우

Command-F2를 누를 때 iMac이 대상 디스플레이 모드로 전환되지 않으면 다음 사항을 확인하십시오.

•   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.

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