How to Fix iTunes Error 21

Solution:

ailbreaking your iPhone, iPod Touch or iPod using custom IPSW made by PwnageTool or Sn0wbreeze requires a full firmware restore using iTunes. During the restore process, if you encounter Error 21, it means that you are trying to restore custom IPSW using the recovery mode. To fix this issue, you must put your iDevice into DFU mode.

DFU or Device Firmware Upgrade mode allows all iDevices to be restored from any state. The actual difference between recovery and DFU mode is that when you put your iDevice into DFU mode, it do not loads iBoot automatically. When you put your iPhone in recovery mode, it loads iBoot which makes sure that the firmware you are trying to restore is greater or equal to the current firmware.
To fix iTunes error 21, you must put your iDevice into DFU mode using the following instructions:

1. Connect your iDevice to your PC or Mac via USB cable and launch iTunes.
2. Hold down the Home+Power button for 10 seconds.
3. After 10 seconds, release the Power button but keep holding the Home button until iTunes detect your iPhone in recovery mode.
4. Once your iDevice is in DFU mode, you should see nothing on its screen. It should be completely blank.

Even if your iDevice is in DFU mode, iTunes will detect it as a recovery mode device and will display a message as shown in the above screenshot. As soon as you see the above message, restore custom IPSW and hopefully this time you will not get iTunes Error 21.

 

Collected from: http://xiphone.org/itunes-fix-error/fix-itunes-error-21