HT4972: iOS 4: Updating your device to iOS 5 or laterLearn about iOS 4: Updating your device to iOS 5 or later
Currently Being ModeratedNov 28, 2012 10:20 AM (in response to tonynguyen6799)
Here is Apple discussion on this error 21:
This error typically occurs when security software interferes with the restore and update process. Follow Troubleshooting security software issues to resolve this issue. In rare cases, these errors may be a hardware issue. If the errors persist on another computer, the device may need service.
After following Apple advice if still Error 21, I would perform a DFU Mode Restore.
Here are the steps. Open iTunes on computer. Connect iPhone to computer with USB cable. Hold both Home and Power buttons, after exactly 10 seconds release the Power Button, keep holding the Home button until iTunes recognizes iPhone in Recovery Mode, usually about 20 seconds time. If the iPhone screen is black, blank this is DFU Mode. Click the Restore button of iTunes. DFU Mode can be difficult to enter sometimes, just keep trying. DFU Mode is a black, blank screen with iTunes saying Recovery Mode.
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