Now this is only a nominal size you can see, similar to how sometimes turns off then switch fixes a few problems on your computer. If there is a real problem, this method will be helpless before, but if the error is an error 21 iPhone while, this should take care of it. You remember how I mentioned that iTunes error 21 is the result of iTunes blocked. Well, sometimes just a restart of iTunes, then begin the process of restore again may prove enough to fix iTunes error 21. So, you may want to ‘ lead with it.