Question: photos app crashes on export
I am trying to export the photos collected on icloud from one library to a folder, to be added to my main library. As far as I have read, I cannot simply download the 36,000 or so photos from icloud into the main library without uploading all of my other photos (I do not have the space to do this on icloud). So I downloaded them all to a separate library. Every time I try to export the photos, the photos app gets to like 35,998 photos reportedly exported, and then crashes. In reality, no more than 6,000-7,000 of these photos are ever actually exported at one time.
My question is, how do I fix this? OR what is another way of combining the two libraries?
I have a 2011 Mac Pro and a 2015 Mac Air, both are running High Sierra. My photos library is on an external HDD that I transfer back and forth. The HDD itself is not disconnecting from the macs, so I do not think that is the problem. I have tried to export multiple times on both macs and while the air reported getting further, it had actually exported less photos than the pro. I have also tried to chunk it to 3000 photos at a time on the pro, to both the external, and then to just the mac's internal HDD and the problem remained the same. Space to the HDDs themselves is not the issue, there is plenty.
MacBook Air, iOS 11.3, or 2011 MacBook Pro