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What does iTunes error 21 mean ?

When you found iTunes error 21, it means that you are attempting to restore custom ipsw in the recovery mode.

How to Fix iTunes error 21 ?

To fix iTunes error 21 on iPhone 4, 3GS, iPod Touch 4G, 3G or iPad you have to restore to custom ipsw when your device in the DFU mode (Device Firmware Upgrade).

Steps to put your device in DFU mode to Fix iTunes error 21:

Step 1: Connect your iPhone to your Mac/PC and press power button for few seconds. You will be asked to slide to power off. Slick it to power off your iPhone.
Step 2: Now Press and hold Home button and the Sleep/Wake button at the same time.

Step 3: After 10 seconds release the Sleep/Wake button but keep holding the home button until iTunes pops up with a message telling you that it has detected an iPhone in recovery mode.

Step 4: Once you are in DFU mode you wont see anything on you screen. It will be BLACK.

Now you have fixed iTunes error 21, go now and restore to custom ipsw firmware without errors