File cannot be found in Develop Module - Photo NOT missing from Library Module

I had this EXACT same problem and spoke with several Adobe experts through their chat and they were unable to resolve my problem. I finally found this page and it solved it:

https://helpx.adobe.com/lightroom-classic/help/allow-permissions.html

"I don't know why or how but the issue seems to have resolved itself by restarting LR and backing up the catalog on exit."

Glad the issue got resolved! Restarting often fixes mysterious behavior issues, unfortunately. It's also possible that the integrity check and optimization normally performed on backup fixed the issue, though that's less likely.

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I note that the selected photo does not show a Histogram, in both Develop and Library.

Certainly a 'glitch' in the Catalog or UI.

Might be worth resetting the Preferences file?

One thing missing in those screenshots, where in the Folders Panel are you keeping those photos? Why can we not se the specific folder highlited (probably off panel view needs to be scrolled up/down?)

Should not the screen look more like this:

In that view options (Library) click on the option to Show Grid Extras.

All those grayed out options are annoying, and may be preventing some info from being displayed.

I suspect you may have imported the photo twice, and now one of them is missing. So now you have to convince me that this is not the case.

Using the filter bar, search for this photo using the following method, performing all 4 steps

1. click on All Photogrpahs
2. expand all stacks (Photo->Stacking->Expand All Stacks)
3. turn off all filters
4. search by file name using the Lightroom filter bar

do you find two (or more) instances of this photo?

The "!" for a missing photo always displays, regardless of the view settings.

LR is clearly confused for some unknown reason (perhaps its fault, perhaps not).  Try doing the menu command Library > Find All Missing Photos -- does that show photos that aren't marked with "!" ?

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You have the Badges Turned Off that would show if LR thinks the file is missing. Turn The Badges On in the Veiw Settings for the Filmstrip and the Grid View.

Also you are showing only the Loupe View in the Library module. Switch to the Grid view.

Even though none of your Folders are showing a question mark all that means is that LR is seeing the folder on the external drive, IE the folder is on the external drive. That does not mean the actual File that is showing as missing is still in that folder. Use Finder to check those image files are still in the proper folders.