How to Sync Movies Without Erasing the Data From the iPhone

When you elect to automatically sync movies with iTunes 11, any videos that are absent from your computer library are erased from your iPhone whenever you connect the device. Configuring manual syncing resolves this problem by offering individual control over which movies are synced without deleting existing iPhone files. This option also conserves limited storage space on your iPhone by not copying large, unnecessary video files.

Open iTunes 11 without connecting your iPhone.

Hold "Ctrl-Shift" (PC) or "Command-Option" (Mac) and connect the iPhone to your computer using the USB data/charging cable. Release the keys after your iPhone appears in the list of devices. Holding these keys prevents iTunes from automatically syncing data.

Select your iPhone from the list of devices and then check "Manually Manage Music and Videos" from the Summary tab. Click "Done."

Open your Movies library and then drag the videos you want to sync to the right. As your mouse cursor approaches the right side, a sidebar appears that lists your connected devices. Drop the videos onto your iPhone label to copy the movies.