Hi Grant (SFA) & gusferlizi. Thanks for your suggestions.
I have tried changing the ICC profile, and the curve; turned off Annotations; tried proofing on and off with various recipes; all to no avail.
I seem to have tied this down to simply zooming in on the image in the editor window, irrespective of whether a crop has been made.
After invoking 'Enable Black & White', changing the profile or the white balance alters the number and position of the white blocks but does not eliminate them.
I have been using Fuji X-Trans sensor (X-T1 & 2) where this has shown up. But checking on some older Canon CR2 raw files, the white blocks also appear but they are there all the time after a B&W conversion, not just on zooming in. Seems somewhat unpredictable re number and position of white blocks.
Latest tests show that even if the editor window is clean, the jpg or psd processed image is corrupt. Please see
https://www.flickr.com/photos/martin_46 ... ateposted/ (jpg and psd results in browser)
I'm relatively new to CO, maybe 6 months, but do quite a bit of B&W work. I've tested this on several images converted to B&W with similar results.
Maybe it's time for new hardware.
Martin