Target Display Mode -- Pressing command-F2 does nothing I would like to avail myself of the target display mode.  I have a brand new MacBook Pro Retnia, and I'd like to use it as a display for my Mac Pro (also brand new).  I followed the instructions on this page, and nothing happened when I pressed command F2.  Then I started scouring the forums.  I found several threads with the same problem, and I tried all the suggestions.  I checked my preferences to see if the keyboard assignments were messed up.  I tried restarting both computers in various orders.  I tried using internal and external keyboards on both computers.  I tried adding the fn key to the key command.  I spent a couple hours on this, but sadly, nothing worked.

So I'm really hoping that someone here has some idea what's going on.  This experience has really been disheartening.  I just bought these computers this year, and they failed at something so basic.  At this point, I would be satisfied just to see an error message telling me why cmd-F2 doesn't work.  I mean, if I try to eject a disk that has files that are currently in use, it will tell me why it can't eject right now.  But with this, I'm completely in the dark.  It's so frustrating!
iMac 27 5K Late 2014 model and later no longer support Target Display Mode.