This was not as easy as it seemed. A checklist to get target display mode working:
- Make sure both your iMac and your Macbook Pro are equipped with Thunderbolt. It only works Thunderbolt -> Thunderbolt or Display port -> Display port, no mix allowed.
- Make sure you are using a Thunderbolt cable, display port cables won’t work for newer models equipped with TB.
- Make sure the cable on iMac’s side is turned the right way (you can insert it upside down into the iMac but not into Macbook Pro!).
- Make sure you are using a Mac keyboard, a PC keyboard will not work, even if remapped. Use the small Bluetooth keyboard provided with your iMac.
- Press Command-F2 on your iMac’s keyboard. If your function keys are not used as “standard function keys” (can be set in Settings / Keyboard) – use Command-Fn-F2.
When in target display mode you can adjust the brightness of your iMac’s screen by pressing F1/F2 on the iMac’s keyboard. To go back to regular mode press Command-F2 again.