Error message:
The iPhone could not be restored. An unknown error occurred
This error is a catch-all for a range of different problems. Depending on the underlying cause, there are a number of possible solutions:
Using the latest version of iTunes (and not a beta!) is a good place to start -- upgrade, if you're not already using the latest version.
Windows users can find a reboot helps to resolve this issue (if you follow the above and upgrade iTunes, you'll need to reboot anyway)
Completely resetting the phone and running a restore in DFU mode
Apple have a short document with further tips for this error
If these tips don’t work, find out how to fix a corrupt backup here .