“Recently my iPhone 6 just reset itself out of nowhere and wouldn't turn back on. I placed it on a dock & the connect to iTunes screen showed up. I plugged it into my computer and when it began to restore it stopped and the unknown error 21 windows popped up. This has occurred on 3 different computers & now I'm unsure how to handle this.”
The most common reason for iPhone Error 21/iTunes Error 21:Download the restore files (.ipsw) but is being blocked from authentication.
Corrupt download or incomplete installation of iTunes software.
Virus or malware infection that has corrupted Windows system files or iTunes-related program files.
Part 1. Troubleshoot Each of the possible causes of iPhone error 21
Part 2. Check Hosts File
Part 3. Fix iTunes error 21 by updating iTunes
Part 4. Fix iTunes error 21 via recovery mode
Part 5. Fix iTunes error 21 or iPhone error 21 without losing data
a. Turn off Anti-Virus to fix iPhone error 21Sometimes, the proper functionality of some vital programs may be hampered by the anti-virus as they might be corrupted or be threats, so you need to turn off the Anti-Virus to prevent it from damage your iPhone.
b. Remove unnecessary USB devicesTo restore and bringing up iPhone Error 21, you may try to remove all unnecessary external devices from the computer.
c. You have to do: Open the device, confiscate the screw which connects the battery. Disconnect the device cable and put it back together.
It seems like quite an extreme and risky measure, so you need to make a decision after careful consideration.
d. Restart iTunes to fix iTunes error 21Sometimes simply restarting the iTunes, then starting the restore process again can prove enough to fix iTunes Error 21.
Step 1. Launch Applications and go to Utility folder
Step 2. Open Terminal
Step 3. Type in the command shown below:
Sudo nano /private/etc/hosts
Step 4. If it asks for a password, type in the password you use to sign into your Mac and then press the Return key
Step 5. The terminal will now display the host's file as shown below. Scroll through the file and see if you can find an entry containing “gs.apple.com”. If you don’t find this then the host file isn’t creating any issues. Otherwise, if you find this entry, then remove the entire line that contains “gs.apple.com”.
Step 2. Go to Menu > Help.
Step 3. Select 'Check for Updates.'
Step 1. Put your device into DFU Mode.1. Hold the power button down for 3 seconds.
2. Hold the power and the home button down for 15 seconds.
3. Release the power button while still holding down the home button for 10 seconds more.
4. You'll be asked to "connect to iTunes screen."
See more: How to Put your iPhone in Recovery Mode?
Step 2. Connect to iTunes.>Connect your iPhone to your computer, and access iTunes.
Step 3. Restore iTunes.1. Open 'Summary' tab in iTunes, then click 'Restore.'
2. After Restore, your device is going to Restart.
3. When asked to "Slide to set up," simply follow the Setup.
However, if you choose this method you will lose all your data because it will restore your iPhone to factory settings. So, you should back up your iPhone data firstly when you fix iTunes error 21 via recovery mode.
Toolkit - iOS System Recovery is a professional tool which can help you fix iPhone Error 21 without losing anything.
iPhone 7 Plus, iPhone 7, iPhone SE, iPhone 6s Plus, iPhone 6s, iPhone 6 Plus, iPhone 6, iPhone 5s, iPhone 5c, iPhone 5, iPhone 4s, iPhone 4 that run iOS 10/9.3/8/7
All models of iPad Pro, iPad Air, iPad mini, iPad
iPod touch 5, iPod touch 4
Windows 10 or Mac 10.11
Windows 10/8.1/8/7/Vista/XP; Mac OS X 10.12 (MacOS Sierra), 10.11(El Capitan), 10.10 (Yosemite), 10.9(Mavericks), 10.8, 10.7, or 10.6
Video Guide: How to fix iOS systems problems and iPhone errors
Step 1. Install and Launch the Program.After downloading the program, then run it.
Step 2. Fix the iPhone error 21.a. Choose "More Tools" and then "iOS System Recovery".
b. Connect your iPhone/Pad/Pod to the computer with a USB cable. Then it will detect your iPhone/Pad/Pod automatically and click the "Start" button.
c. You're now well on the well to successfully dealing with Error 21, then click "Start" button.
Step 3. Download the firmwarea. The drop-down menus allow you to easily and accurately choose your model.
b. Choose whether iPhone or Pad or Pod, then the specific model, and so on.
c. The right firmware will now be downloaded and automatically installed on your iPhone/Pad/Pod.
Step 4. Fix iTunes error 21Once the download is complete, iOS System Recovery will automatically start repairing the iOS, except this time you won't be bothered with an iPhone error 21 message!
You May Like: How to Fix iPhone Error 3014?
To fix iPhone Error 21, here you may see different methods with their advantages and disadvantages. Just choose the one you need to do it if you are also encouraging it.