Photos app failing to export & refusing to drag & drop
This problem is happening in the Photos app on a client's MacBook Air running 10.13.6.
She has an album of selected holiday snaps that we've been trying to export so she can share them with friends. Her Photos library is stored on an external SanDisk SSD (500GB, HFS+) because the internal drive on the MBA doesn't have enough space.
When we select the full 802 photos in the album then try to drag & drop onto the desktop the cursor appropriately says that all 802 will be deposited. But once that operation has started if we click on the export progress "pie chart" icon it tells us that just 122 photos are being exported (and if we let it go to completion that is indeed all that are copied). If we subsequently retry any drag & drop operation within that same session of Photos, whether it be all 802 or 1 or 2 or 15, the Photos app mostly (but not always) refuses to acknowledge the "drag". ie. no change of cursor & no action performed when doing the "drop".
If we use the File > Export > (modified) method and choose high quality jpg with the 802 photos selected the operation initiates and the progress bar indicates 802 photos being processed but when it gets to around 250 (not consistent) it would seem to get an error of some sort and quickly pretends to process the remainder (but we again end up with only a partial set) and the Photos app then crashes. I've taken a quick look at the crash report but nothing in it jumps out at me and the fact that it doesn't crash until after the pretence of processing the remaining photos makes me think it might not have any useful context anyway.
These symptoms exhibit for both the desktop and an external USB as the destinations. I have run the built-in repair function on the library with no change to symptoms.
I used FatCat's Power Photos to create a clean Photos library on the external SSD and used its drag & drop to successfully copy the 802 photos into that test library. (Note: Power Photos actually works by sending commands to the Photos app.) However performing the drag & drop or export from that test library exhibits the same symptoms!
I then tried the File > Export > Unmodified originals method and, although it took a long time, it "processed" all 802. But to add a twist.... in the destination folder Finder initially reported just 800 items and then after a couple of minutes (while I was talking to my client) it changed its mind and said there were now 801 items.
So, I can't come up with a consistent idea of where the error is living. Is it a particular corrupt photo that then corrupts the Photos working memory, is it the library indexing structure, is it a bug in the Photos app, or is it some strange symptom related to the library being on an external (HFS+) SSD?
We do want to have an answer but I also have to be mindful that I've spent several hours on this problem without success and can't charge my client for all that time and taking control of her machine for another couple of hours of testing exacerbates that unpaid work ratio. (And some of the testing options aren't available as I'm having to do this remotely, via TeamViewer.)