iPhone4 won't restore and shows Error 21
My phone won't turn on, but shows the Apple logo when I plug it in (either to the wall charger or the computer). Then, it goes to the Connect to iTunes screen and when I connect it to iTunes it tells me that I need to restore the phone. It will get all the way to "preparing iPhone for restore" then will say, "The iPhone could not be restored. An unknown error occurred (21)." I tried to disable my computer security, but it did the same thing. Does anyone know what else I could do to fix it? Please help!