Q: I have a late 2012 27" iMac that I want to use as a monitor for a Surface Pro 3.  I have tried using a Mini DisplayPort cable between the 2 devices but that hasn't worked.  Does anyone have a solution?

Sorry to be the bearer of bad news but it simply will not work. What you are describing is called Target Display Mode, Apple discusses this in Use your iMac as a display with Target Display Mode - Apple Support. The part that describes that it will not work is"

"To use your iMac as a display, connect it to another Mac using the right cable (see System Requirements below). Then press Command-F2 on the keyboard connected to your iMac. When you're done, press Command-F2 again."

Please do not ask if there is any other method to make what you want to work, unfortunately it does not exist.

Unfortunately, there is no way to do this. As explained above, Target Display Mode only works between Macs.  And certain Macs at that.

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