Related errors: 2, 4, 6, 9, 1000, 1611, 9006, 9807, or 9844. Sometimes as a result of this issue, a device might stop responding during the restore process.
Check your security software and settings, which can block ports and prevent connection to Apple servers during update and restore.
Update and restore alert messages on iPhone, iPad, and iPod touch
http://www.buybuyla.com/tech/view/012953a0d412000e.shtml
iOS: Resolving update and restore alert messages
http://support.apple.com/kb/TS1275
iPad: Unable to update or restore
http://support.apple.com/kb/ht4097
iTunes: Specific update-and-restore error messages and advanced troubleshooting
http://support.apple.com/kb/TS3694
If you can’t update or restore your iOS device
http://support.apple.com/kb/ht1808
Cheers, Tom 😉