Initial previews not created on downloading images

When downloading images on my imac with an SD card inserted, the images are imported but no previews are created, leaving only a gray file. I can see the images one at a time by going to the develop module, so I know they have been downloaded. I'm using lightroom classic, 12.5 release on a 27" imac 2015 1/2 version. Images are being saved to an external WD 4Tb external drive that has not had any previous problems. I discovered this after returning from a vacation trip and have not been able to solve it. 

Since working on this currently, I have discovered all the previews are missing for my existing files, although the edits are in place, that I can see in Develop.

I have also imported some of the same images to my macbook air with no problem. Maybe there is something I am missing here, but suggestions would be appreciated.

Milton McLain