I have a late 2009 27" iMac Core 2 Duo running 10.6.8 Snow Leopard that I would like to use as an external monitor for my core i7 15" mid-2012 Macbook Pro (os x El Capitan) with a NVIDIA GeForce GT 650M 512 MB graphics card (not a retina display). The highest resolution for the macbook is 1440 x 900. When I use my iMac the resolution is 2560 x 1440 but when I hook it up to my macbook with mini displayport cables in Target Display Mode I can only view the display in 1920x1200, 1680x1050 or 1280x720. Also the screen is very pixelated, especially when it’s scaled to 1920 x 1200. Is there a way to view the 27" in 2560 x 1440 or will I have to buy a smaller external monitor?