HT203843: Resolve USB-related alerts in iTunes
Learn about Resolve USB-related alerts in iTunes
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Helpful answers
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Feb 16, 2014 11:13 AM in response to fernlewisxby Lawrence Finch,See this troubleshooting tip: http://support.apple.com/kb/TS3694#hardware
Note that frequently error 21 means that the phone was jailbroken or hacked.