Hi, I'm very much wet behind the ears on this topic and would appreciate some sound advice! I have a 21.5" intel dual core 2009 iMac that I would like to use on occasion as a mirror display for my 2012 MacBook Pro. I would like to mirror the mbp screen on the 2009 imac. Not sure if this can be done??? I'm not sure which cable or adapter is needed to make this happen? Also are there any settings I would need to amend on either system to make this work. Thank you! 2009 imac, 2011 MBA, 2012 MBP with retina, IPhone 4, IPod Classic & Ipad 3.