A few days ago, my iTunes started giving me messages like “iTunes could not sync contacts to the iPhone “My iPhone” because the iPhone disconnected.” It wasn’t syncing anything to the iPhone anymore.

After reading this forum thread I tried resetting the sync history in the iSync application preferences. This did not help and instead caused iSync reminder messages to come up every few minutes.

I also tried syncing the iPhone with all the “info” selections unchecked, and then finally replacing all info on the iPhone by checking “Replace with information from this computer” in the Info tab in iTunes. This last change seemed to work, but I realized later that something else had probably helped fix it:

I’d had a USB hard drive plugged into the same USB hub as my iPhone for the last few days, and when nothing else was working, I finally shut it down and unplugged it. So there was more power for the iPhone coming from the hub. It is a power issue.

A couple of other troubleshooting guides to help you if you are having similar problems:

  • Troubleshooting iPhone and iPod touch calendar syncing via USB on Windows:[LINK]
    • Reset Sync History
    • Disable third-party Outlook add-ins
    • Attempt to sync content in a new Windows user
    • Reinstall the iTunes software
  • iPhone: Troubleshooting USB connections [LINK]
    Symptoms:
    When attempting to sync, update, or restore the iPhone, you may receive one or more of the following symptoms:

    • An alert message indicates an unknown error occurred
    • The iPhone unexpectedly disconnects during a sync or restore
    • Photos downloaded from the iPhone appear garbled or not all photos transfer
    • An alert message indicates the USB device cannot operate because it needs more power than is available
    • An alert message indicates “this device can perform faster” if you plug the device into a higher speed USB port”