I'm trying to move all my photos into one Photos album and I'm having issues.
All the photos have been exported out of either Photos on an iMac or from Lightroom and put on an external hard drive. When I drag the photos into Apple Photos on my MacBook Pro, I'm being told:
Cannot Import Items
Some items cannot be added to your Photo library because they may be an unrecognizable file format or the file may not contain valid data.

This wikiHow teaches you how to copy an iPhone's photos onto a Mac computer. Monitors the work from our. If you want to import all photos that aren't. A little-known use for the Mac Preview app is to import photos from an iPhone, camera or SD card. This functionality is similar to using Image Capture, but coupled with Preview's ability to view and edit the photos as well. It doesn't work perfectly with iCloud Photo Library, since it is hard to know which photos are actually on your iPhone at any given time.
I'm running a maxed out Late 2013 15' MBP on 10.13.1.
I've tried fixing permissions, and importing in smaller batches, to no avail. There are about 20,000 photos and this warning is appearing for about 17,000 of them.

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