I had this problem with black borders too (0.5" at the top and bottom and 1.5" on the left and right sides), tried different mini to displayport and hdmi cables, searched for answers (which were sparse), nothing worked. I am using a MacBook Air 2015 13". On two monitors, LG34UM95, and LG34UM68, the exact same problem was replicated on both screens. The resolutions were showing as 3440*1440 and 2560 *1080, but it was the same black borders all around. I was desperate and thought it's a problem with my cables, El Capitan, or macs not working with LG monitors.
Underscan option only showed up on hdmi but was still not putting out the full resolution (1920px max width) - desktop was not filling the whole monitor screen, just half of it. Well, we actually don't need to adjust this setting at all.
Here are the steps that worked for me, using mathewwinkson's instructions to force RGB mode in mac osx to fix the picture quality of an external monitor. After following all the steps and restarting for the final time, I was amazed - there are no longer having black borders around my desktop image on the monitor! Finally, I can enjoy the full quality of this beautiful widescreen monitor.
Make sure that you are in the system library folder, not user library folder. Cheers.
http://www.mathewinkson.com/2013/03/force-rgb-mode-in-mac-os-x-to-fix-the-pictur e-quality-of-an-external-monitor#comment…
