As you can see, a 27-inch iMac (yours has got Mini DisplayPort) can be used as an external display for any other Mini DisplayPort or Thunderbolt Mac, as your new iMac. You just need a Mini DisplayPort to Mini DisplayPort cable (note that the Late 2009 iMac can't detect a Thunderbolt cable) and put the Late 2009 in Target Display mode, following the steps of this page
Thank you. Here's where my confusion comes in
"iMac (Mid 2011) and later: target Display Mode does not work with Mini DisplayPort cable"
When you attempt to connect an iMac (21.5-inch or 27-inch, Mid 2011) and later to another computer with a Mini DisplayPort to Mini DisplayPort cable, the screen of the target iMac will not become an external display when you press Command-F2 to activate Target Display Mode.
iMac (21.5-inch, Mid 2011) and iMac (27-inch, Mid 2011) and later computers support Target Display Mode via Thunderbolt to Thunderbolt cable (2 m) when the source is another Thunderbolt-equipped computer.
iMac (27-inch, Mid 2011) computers do not support Target Display Mode via Mini DisplayPort cables.
It's confusing. In my opinion, if you aren't sure if it will work, you can go to a different solution: use ScreenRecycler. This app allows you to use a Mac or PC as an external display for another one wirelessly, without connecting both Macs or PCs, and you can be sure that it will work, because you just have to connect both Macs to the same network
For the record. Got the Mini DP to Mini DP cable and it worked flawlessly. Connected both 27" iMacs while on. The older (later 2009) immediately went to an extended screen of my new (late 2012) iMac. Command-F2 on my older machines toggles between "Target Display Mode" and normal operations.
Thanks mende1 for your advice! At least 3 "so called experts" at various stores said it wouldn't work.
Hi there. I am in the ecxact same situtaion as yourself. I have a late 2012 iMac as my main computer and also a late 2009 iMac that I wish to use as a second monitor for added screen real estate when using Logic Pro X etc. I've just purchased the MDP to MDP cable. When I connect the lead my late 2012 iMac screen is going black and becomes unresponsive (CMD F2 not doing anything). Is there anything you can suggest to help me get the late 2012 to become the primary display and the late 2009 as the extra display? Am I missing anything? Any help or light on the matter is greatly appreciayted. Thank you