I don't think these are missing photos for two reasons:
1: None of the photos in the film strip shows the missing photos exclamation mark.
2: Yes, there are missing folders on Macintosh HD, but none of these folders is selected. We cannot see the selected folder, but it seems to be a folder on the My Passport for Mac disk.
To the OP: did you recently upgrade MacOS? In that case, check that Lightroom has the necessary permissions:
1: Your previews cache is corrupted. Move 'catalogname previews.lrdata' out of the catalog folder and then start Lightroom. Lightroom should now start to rebuild the previews.
2: This is caused by a corrupted monitor profile. Select a different profile (any profile) in System Settings - Displays to see if that solves it.
There's a third possibility that has struck a few in the past -- "zombie" thumbnails.
3. Click on one of the empty thumbnails. In the Metadata panel, do you see the standard fields like File Name, Dimensions, Camera, etc. or are they all blank?
If you're not sure, post a full-resolution screenshot of your entire LR window.
Yes, I thought about that one too, but I never heard of a situation where all images were zombie thumbnails. Usually there are only a few dozen, and the real images do have normal previews.