Photos in Mac show as red rectangles
Several of my photos in the Mac app Photos are showing up as bright red rectangles, whereas most photos are showing the...photos. Some are from my iPhone and some are from my Canon 50D. There doesn't seem to be any rhyme or reason. Before the last MacOS update, the red photos weren't visible. Now, most will show after I select them. Any ideas?
iMac, macOS High Sierra (10.13.4), 21.5" mid 2011 12 GB Ram