I got an new iPhone 5S and preparing to sell my old iPhone 4S to someone else. To protect my personal data being leaked, I was told to remove all the personal data before selling it. I tried the default method by Apple Support Team: Settings->General->Reset->Erase All Content and Settings. When I clicked the Erase iPhone option from the promote, nothing happened in an hour, only a grayscreen flashing all the time. I have done this several times and the problem still exsits.
Does anyone have workable solution to fix this problem or any recommendation? Thanks in advance.
My Device: iPhone 4S with iOS 7.1.2