I love the way iTunes, Photos and my iPhone sync together. It works and is a great way of ensuring all devices are up to date with the latest information. There does seem to be a reoccurring problem with iTunes and Photos library and the inability to sync photos correctly. Once in a while (which seems to be getting more common) iTunes will fail to sync photos. The underlying problem seems to be due to a permissions error which is easy to fix.
To solve this problem, you first need to navigate to the source of your Photos library. Open finder and locate it, typically within your User area under Pictures.
Right click on ‘Photos Library’ and select Get Info. At the bottom you will see a lock. Click it and enter your admin password. From the drop down lists at the bottom ensure ‘everyone’ has the option to read and write to the file.
This will allow the system and other process to write to your Photos library. It seems during the sync process if iTunes needs to write to the library it can’t, thus the process fails.
Restart both Photos and iTunes and sync you phone again. It should now work.
I don’t know why this fails, but this fix does seem to work.