I've wondering if someone can help me with an issue. I'm trying to use my new iMac in target display mode with my wife's 13" Retina MBP. I plug them both in via thunderbolt, I hit cmd-f2 and the rMBP recognizes the iMac as a display, however, the iMac is just blank (all black, no picture) I think I'm doing everything right for this to work. All thunderbolt, both new machines, used cmd-f2. The 13" rMBP can handle a second monitor and the new iMac can handle target display mode (via thunderbolt machines only). Why isn't this working? Why is my iMac screen blank?