First make sure you backup your data in /Music/iTunes on an external HD or a USB drive
Then,
Launch Terminal app /Applications/Utilities/
Kill iTunes app and processes by typing killall iTunes;
killall "iTunes Helper"
- Type this command to delete iTunes:
sudo rm -rf /Applications/iTunes.app/
- Type in your password which won't be shown
- Quit iTunes and go to to /Applications, click once on iTunes
- Hold down CMD + I or right click on it and choose “Get Info” from the menu
- Expand “Sharing & Permissions”
- Authenticate your account by clicking on the lock icon in the lower right corner
- Under “Privilege” set “Everyone” -> “Read & Write”
- Close the Get Info window and now drag iTunes to the Trash
- Empty the trash
Now iTunes is completely uninstalled. Download it again from Apple, be sure your devices are running the latest version of iOS (6.0) and try to sync.
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If the process above is not working, try to reset your iDevice.
Download your iDevice firmware by typing in Google, i.e. iPhone 4 iOS 6 download.
Store it in your Downloads folder or Desktop.
Put your iDevice in DFU mode:
- Plug your iDevice to your Mac \ PC
- Turn it off
- Hold down Sleep button and Home button for about 10 seconds
- Release Sleep button but keep holding down Home button until iTunes recognizes a device in Recovery mode.
- Hold down OPTION (Mac) or SHIFT (Windows) to restore from .ipsw
- Select your previously downloaded firmware and wait for the operation to complete.
I HIGHLY recommend to set the iDevice as a new one and not restoring it from a backup (this will avoid general slowness issues or various incompatiblities). But, if you really need it, restore from backup and try to see if the sync works.
I take no responsibility for anything that may go wrong during the following or mis-following of any processes listed above.