There may come a time when your iPhone is so uncooperative—it’s crashing constantly or freezing and you have to press Sleep/Wake and Home for several seconds to bring it back to life—that it needs to be restored. Or you may choose to restore your iPhone when you’ve hacked your phone and then wish to install the latest Apple iPhone update.
There’s the very unlikely possibility that during that restore attempt you’ll see this error message: “The iPhone could not be restored. An unknown error occurred. Error 1603.”
To fix this problem, restart your Mac and try again. If that doesn’t eliminate the error, connect your iPhone to a different USB port, ideally one that’s connected to the Mac itself (rather than to a keyboard or USB hub). And if that doesn’t work, some users have reported success by unhitching the iPhone from their computer, removing its SIM card, plugging the iPhone into the computer, and then clicking Restore. Once the phone has been updated restore the SIM card.