To clarify, the videos have blank grey thumbnails. Most of the videos that seem to have problems are GoPro videos and it seems the corresponding GoPro thumbnail file (which LR can't read) is in the folder with them. It also seems that many of the jagged thumbnail files came from my iPhone dump.
Which version of Lightroom do you have?
Do those photos and videos show an exclamation on top right?
Do you have those photos and videos at their original location on your hard drive?
I believe those photos and videos appear to be missing.
If possible, share a screenshot of your Lightroom grid module with those photos and videos
They were originally entered in LR5 I believe but I'm now upgraded to LR6. I believe the index was upgraded when I opened this set of photos.
It seems that the complete folder structure on your 3TB is missing in Lightroom.
Do you still have all those images and folders on your LEC G-SAFE 3TB hard drive?
would suggest you to relocate those images.
You can follow this article for details step to locate those missing images :
That worked. Thank you very much.