iMac display issues...

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My iMac is a 27-inch, Late 2013 model with a 3.5GHz Intel Core i7 (Haswell), 8GB 1600MHz DDR3 RAM and NVIDIA GeForce GTX 775M 2048 MB graphics. From one moment to the other, it decided to display this:

 

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What should I do?

 

[Erdbeertorte]

Do you see it all the time or does it appear just sometimes?

If your iMac has this option, does it also happen when you disable it?

 

[497902]

It appears all the time, even after I reinstalled the OS. No, it doesn't have that option...