Second monitor not displaying correctly?

If you are using display mirroring (when the same view is on both your laptop screen and the external monitor), this is normal. It is because your laptop's display is using the 16:10 ratio, and the monitor you plugged in is 16:9, so it would be impossible to mirror the image without introducing black bars somewhere.

Turn off display mirroring, or close your laptop's lid and use it in clamshell mode, and it should fix itself.

 

-Already suggested-

 

can you do an 'auto detect' on your monitor buttons, it can usually fix these issues. Also make sure it's the native res for your external monitor.

 

You could try using the under scale option in system prefs > display. It's a slider bar. Another thing you can try is adjusting the settings on the external display. My TV I'm using originally cut off the edges when I connected my computer, but after changing the "zoom" setting to "dot-by-dot" instead of "full" which I normally have it on for TV, it fixed the problem.

 

I had ben facing a similar issue when using my 21.5" Alienware monitor on my Desktop via HDMI. Even though the resolution was set to 1080p, the actually display area was smaller. Shifting to DVI switched the issue. Perhaps you could try switching to HDMI?

Are you using the same monitor or a similar Dell monitor? Did you try another display using the same setup or the same display on another setup?

Regards
Raptor

 

Curious, is this when driving two displays, or if you close the lid on your MBP will the external display then display correctly?

I've seen mismatches in dual display with retina MBP's and external monitors, and I've seen cheapo DVI converters both cause this problem.

Keep in mind when doing multiple monitors that the resolution is technically the total of the two resolutions so 2560x1440 x 2 really is a single GPU or onboard processor graphics driving the equivalent of 5120x1440 in a single monitor. I deal with it all the time with users that have virtual desktops pushed down from a server and try to drive two monitors. Others just sometimes go to crap when you try to drive the monitor in a non-native resolution, which judging by the pic/shot is my guess.