How to Fix Error 21 on an iPad


When you update or restore an iPad, iPhone or iPod Touch by connecting it to your computer, you may run into an Error 21 notification from iTunes. Error 21 is one of a dozen error numbers that vaguely point to some kind of hardware problem. The issue could be a bad USB port or a faulty cable; even outdated software can cause it. There is a chance that your computer's security software or a firewall is blocking access to iTunes; troubleshoot this possibility using iTunes Network Connectivity Tests.

Troubleshooting Error 21 Issues

  • Make sure you're logged in to your computer with an administrative account. If you're not certain, log out and then log in with an admin account.

  • Update to the latest version of iTunes on your computer by clicking the iTunes "Help" menu and selecting "Check for Updates." If you don't see the Help menu, press "Ctrl-B" on the keyboard to reveal the Menu bar.

  • Update Windows. If you don't have Windows' default automatic updates enabled, install any updates manually. Press the "Windows" key from the Start screen, select "Settings and then "Change PC Settings." Select "Update and Recovery," followed by "Windows Update." Click "Check Now." If there is an update available, follow the onscreen prompts to install it.

  • Confirm that any security software you installed on your computer is up to date. Windows security software is updated when you do a Windows update.

  • Unplug all USB devices connected to your computer except for the keyboard and mouse. Examine the iPad's Lightning cable to ensure it is in good repair and then plug it in to a USB port you know is working properly on the computer .

  • Restart the computer and the iPad. Launch iTunes and try to update or restore the iPad.

Testing iTunes Network Connectivity

  • Launch iTunes and select "Run Diagnostics" from the Help menu.

  • Select "Network Connectivity Tests," "Device Connectivity Tests" and "Device Sync Tests" by clicking the check boxes. Clear the "CD/DVD Drive Tests" check box.

  • Click the "Next" button. Follow the onscreen prompts and click "Next" again. For example, iTunes prompts you to confirm your computer is connected to a network before it begins the test.

  • Read the test results when they appear on the screen after several seconds. Items with a green status light are working well. Any item with a red or yellow status light needs attention.

  • Click the "Help" button that appears beside items marked yellow or red and follow the onscreen instructions. For example, if your Windows Firewall is blocking access to the Internet, the online instructions guide you in opening access to iTunes.

  • Restart your computer and the iPad and try updating or restoring the iPad two more times. If you still get the error, contact Apple Support or your closest Apple Store.

Tips & Warnings

  • Apple iTunes needs access to ports 80 and 443 on any firewall installed on your computer or network. If you manually configured a firewall to restrict access to these ports, open the access to the eight domains iTunes uses to update its software. These domains are:,,,,,, and
  • Information in this article applies to Apple iOS 7, Windows 8.1 and iTunes 11. It may vary slightly or significantly with other versions or products.

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  • Photo Credit ErikaMitchell/iStock/Getty Images
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