I know (100s of times already...) that this doesn't work. I just don't understand why it doesn't. Most suggestions are that thunderbolt does not yet have enough bandwidth to support 5k information. If this is the case, then why is TDM completely disabled, and not just so for 5k resolution? Or, why am I unable to use TDM under the same limitations of the cheaper, "standard" iMac (or like my first iMac, 2009)? I found the fine print at https://support.apple.com/en-us/HT204592: "iMac (Retina 5K, 27-inch, Late 2014) and later iMac models can't be used as Target Display Mode displays." It seems future iMacs will not include TDM. Is this encouraging TB Display sales? Will the price of iMac decrease accordingly? I believe the retina iMac can be used as an external display at less than 5k. I just don't know the workaround..