Q I am running a recent MacBook Pro 15” model. The image on its internal display is fine, but when I connect it to a 17” external display there are strange black areas, smooth but of irregular form, that appear at the edges of the image. I have ignored this for some time, but they interfere with my ability to read the affected parts of the display. What is causing this, and how can I correct it?
A This appears to be a graphics card issue.
Check first that your MacBook Pro is fully up to date for firmware and other updates. If that is the case, then check whether the same visual flaws occur on other external displays, in case it is specific to this screen, and at each available display resolution.
If it is consistent and reproducible, you should report it via the AppleCare support desk. This is because there are a number of known issues with MacBook Pro graphics cards that are covered by warranty extensions, and it may be that this is one of them.
Updated from the original, which was first published in MacUser volume 27 issue 15, 2011.