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Because the Navigator image shows the same as the main window, it is most likely image file corruption.
Common causes are mostly hardware related- Camera Card (even camera itself), usb cables, card readers, hard-drive failure, RAM memory faults, etc. Try downloading using different methods to compare results.
If the suggestion to "disable GPU support" is un-successful then have a read of this forum thread and others with similar problem-
Be aware, that if the photo looks ok in other software, it is because they may be just showing the embedded (jpg) preview. It will be the raw data in the file that is corrupt.
I join the answer of wobertc. Check all your cables, card.
Check if your picture is correct when viewed in your explorer of finder.