Have this setup giving me fits and wondered if anyone else is experiencing the same issue. And how to best troubleshoot it. Apple says it's supported. Here's what I've tried so far: Display to each thunderbolt port on the iMac: Right display doesn't work Right display to iMac with crossover thunderbolt cable to left display: Neither external monitor works Left display to iMac with crossover from left display to right display - Both monitors work.. UNTIL machine sleeps at which time right monitor won't wake until iMac is cold booted. Warm boot display stays dead. All parts, cables, etc are Apple, so no third party silliness. All firmware latest, either monitor using it as a single works fine, or using it as a single external to the iMac works fine also. Appears to be tied to the dual setup related to the right monitor. What I can't find clear docs for though are whether I should be able to plug each of the Thunderbolts to each of the 2 iMac Thunderbolt ports and have them work (It doesn't). It looks like it's supposed to? Best I've found is @ http://support.apple.com/kb/ht5219#18 Where it says "iMac (27-inch, Mid 2011), iMac (21.5-inch, Late 2012), and iMac (27-inch, Late 2012) with two Thunderbolt ports supports a total of two Thunderbolt displays regardless of which Thunderbolt port each display is connected to" Either monitor hooked to something like my Mini or my MBP work just fine as a single monitor. Ideas appreciated.