Retina MBP using iMac as External Display not working

[mackry]

I have my Retina MBP connected via Thunderbolt cable from Apple to a 2011 iMac with Thunderbolt ports. Both machines are on. When I press CMD + F2 on the iMac to change the Target Display Mode, nothing works. I've tried restarting both machines, doing a SMC reset – still nothing.

Any ideas? Thanks!

 

[JediMeister]

Did you install all of the available software updates for the rMBP before trying to use Target Display Mode? See this.