I have 2 Mac Minis running 10.9.5 and 10.10.5 in with my monitor in landscape mode I have options for all the resolutions that the monitor can handle. If I rotate the screen settings in display it only offers 1024 x 768 and displays a landscape window on the portrait monitor. There is no way of getting it to run at 768 x 1024. I also have a Mac Mini (10.6.7) which will run at 768 x 1024 on the same monitor with no problems.