I had the same problem. The problem is caused by cache and directory issues with the startup disk. BOOTING IN SAFE MODE SOLVED IT. Boot in safe mode (hold down shift key immediately you power the macbook, keep holding it until you see the login screen). To confirm that you are in safe mode (safe mode should be written in red at the top right corner of the login screen). Login, plug in your iphone and the problem is solved. To confirm that the problem is solved, leave safe mode by just restarting the macbook normally (without holding shift). Sorry for the late response. THIS IS A 100% FIX