Trouble importing photos into Photos

[Oleskole]

I'm a little late to this thread, but I had the exact same problem with files not importing because of "unknown error". Since the error was not in the files themselves, but on the computer somehow, I tried to change the user on my iMac, and for some reason it worked. On the other user I could import the files with no issues. My library is on an external hard drive, so the switch was easy.