Import images disappear in a flash

I recently imported many image folders from my PC to my MacBook Pro via a LaCie external drive. I then transferred them to my WD Passport external drive. BUT, when I try to import images to Lightroom Classic, I'll see the usual thumbnails with check boxes for an instant, then they disappear! It doesn't matter if I try this from the external drive or my hard drive, the same thing happens. This only happens with the former PC images.

Although this is probably incidental to the problem, I did click on back up after the PC files were added. After several days and only a fractional import, I canceled out of the back up. Thanks in advance for the help!