Importing photos from iphone to computer error

I have 645 pictures and videos I'm trying to import from iphone to computer and it will do a few photos and the errors pops up and quits downloading if I restart it pops up error. I have to wait unplug phone then a couple minutes later it might import a few might not at all. I have tried doing 8 or just a few photos it pops up error. Any suggestions what I'm doing wrong?