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Uncheck Visualize spots on the toolbar below the image.
The purpose of this feature is to make it easier to identify dust spots.
You may inadvertently have pressed the A key, which toggles Visualize spots on/off.
Note to moderator: Please change to Discussion
That fixed it, thanks Per Berntsen!
Thanks... I've been working in that mode for months, on the assumption that the Mac was out of memory. Additionally, the tool bar was missing, so it wasnt obvious that I could change the visualisation setting.
FYi: In 'develop mode' you can toggle the toolbar on and off using the letter 't'.