Out of interest how do you use your iMac as an external monitor? I have a slightly older one, so it doesn't support target display mode (only available on 2009 macs and newer).
It has a mini display port in the back, and I just run a mini display port cable from my laptop to the iMac. I expected it to be complicated when I was looking into how to do it, but it turned out ridiculously easy, I just needed a $5 cable.