itunes music

What it says, if you proceed to sync any media already on the device is going to be removed and replaced by whatever you've selected from the local library. When you move computers you would ideally follow the advice in something like Move your iTunes library to a new computer - Apple Community to take your iTunes library with you. If you have content on the device that isn't in your iTunes library see Recover your iTunes library from your iPod or iOS device - Apple Community. Once you have it all in one place, and preferably backed up too, you can go ahead and allow the content on the device to be replaced.

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See Sync your iPhone, iPad, or iPod using your computer - Apple Support.

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