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This is a really bizarre problem. I'm using a MacBook Pro running Mountain Lion and an iPhone 4S, both with the latest updates. I'm using the bundled Apple cord. When I plug it into the wall charger, it charges fine. But when I plug it into my MacBook to sync, it will frequently disconnect (sometimes as many as ten times in as many seconds). I jiggle the cord on both ends, plug it into the other USB port, and even cleaned out the inside of the phone. Also, when I use the USB ports for my external keyboard and mouse, everything works fine.

Does anyone have any suggestions?

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Do other devices work on the USB port your iPhone was on? I'd say get someone else's computer, cable, and iPhone and try different combinations of them to isolate the problem. –  Kevin Chen Jan 16 '13 at 7:17
    
Yeah, as noted above I'm typing on an external keyboard that uses the same USB ports. –  Andrew Min Jan 16 '13 at 14:09
    
Same problem here. iPhone 4S (iOS 7.0.4), OS Maverics X 10.9, Macbook Pro 15 (Late 2013). The problem exists only when I'm using left USB port. I have searched trough Apple Forums. A lot of people suggest faulty cable, but I think this is not the case (why it would work on the other port then?). For one user problem was gone with motherboard replacement. –  user71269 Feb 24 at 14:20
    
If you open Console.app and show all messages, do any USB type messages flow by when you plug in the affected device? My guess is that the USB chipset on the computer is sensing too much power and shutting down the port (or trying to shut down the port and failing). –  bmike Feb 24 at 15:03

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