How to solve Mini DisplayPort to HDMI Overscan problem? (Part of the screen is missing)

Solution 1:

There should be an "Underscan" adjustment in display preferences. Open up the Displays in System Preferences. Have a look at your available resolutions. There should be a 720p, 1080i and 1080p, choose whichever is appropriate for your TV, and a slider labelled Underscan should appear. Adjust as necessary to get the picture right on your TV.

Solution 2:

I had this same issue and spent ages trying everything to correct it. Turned out it was just the zoom option in the picture settings of my TV menu. It was set to "Auto" and after changing to "Full" I now have my external display set up perfectly :) I guess the moral of this is to try the simple solutions first!

Edit - There was a few other options in this menu that helped improve the overall quality also, such as turning "HDMI true black" ON