De aqui , a ênfase é minha:
Products Affected iMac (21.5-inch, Mid 2011), iMac (27-inch, Mid 2011)
The iMac (Mid 2011) has Thunderbolt ports that support:
- [...]
- The Thunderbolt port does not accept input for Target Display Mode via a Mini DisplayPort cable.
De aqui :
If you have an iMac with Thunderbolt, any other Mac with Thunderbolt can use it as a display. The iMac will play both the video and audio from the other Mac.
If you have a 27-inch iMac with a Mini DisplayPort, any other Mac with a Mini DisplayPort or Thunderbolt port can use it as a display. Make sure you have a Mini DisplayPort to Mini DisplayPort cable.
Como você tem um Thunderbolt Mac, não pode usá-lo como um monitor para a saída DisplayPort.