Photos app for macOS seems to have known bugs. Examples: here, here, here, here. Some bugs like photos going into 'not responding' mode; photos app continually finding 'new items'; and photos not believing it has imported photos when it had, can be worked around by reattempting the import – by simply force quitting photos and restarting it, then clicking on 'Import all new items'.
The problem is, after importing said new items, the 'Import all new items' turns blue and clickable again. So photos app never seems satisfied that it has received all the imported items.
I will be deleting the drive of the device from which photos are being imported, so it is absolutely essential to be certain that all files have indeed been imported.
Question
How can I know with certainty that all the photos on a device have been imported successfully to photos on macOS?
Best Answer
Sidestep the issue - use Image Capture.
If you plug in any phone or SD card etc it will display the contents.
I set mine to delete on import. Import can be to a folder, or to Photos etc.
You can watch the list of photos go down as they are imported.