The alert message in iTunes may include Error -1 as “This iPhone could not be restored due to an unknown error (-1)” causing problems with restoring your iPhone. Just follow the following tips and you should be through restoring your device in no time.
According to apple, “Error 1 or -1 indicates a hardware issue with your device. If the errors persist on another computer, the device may need service.”
1. Update to the latest version of iTunes
To see if you have the latest version of iTunes, open iTunes and check for updates.
Windows: Choose Help > Check for Updates
Mac: Choose iTunes > Check for Updates
Install any available iTunes updates. If you do not have an active connection to the Internet, or if your Internet security settings are configured to prevent iTunes from communicating with Apple, iTunes will alert you that the update server could not be contacted. Resolve your Internet issues and attempt to update again.
If you are using Windows, see iTunes for Windows XP: iTunes cannot contact the iPod, iPad, or iPhone software update server and iTunes for Windows: Network Connectivity Tests. If you are using Mac OS X, see Mac OS X: Connect to the Internet and troubleshoot your Internet connection.
2. Verify that your operating system is up to date
On a Mac, choose Software Update from the Apple () menu and install all available updates. If an update requires a restart, check for updates again after restarting. To get the latest updates on a Windows PC, go to the Microsoft Windows Update page. If an update requires a restart, check for updates again after restarting.
3. Disconnect other USB devices
Other USB devices may interfere with the communication between iTunes and the iOS device. Follow these steps to resolve an issue caused by other USB devices:
Disconnect all USB devices (hubs, printers, scanners, dangling cords, card readers, cameras, and so on).
Connect only your keyboard and mouse. Connect the iOS device directly to the computer using the white Apple 30-pin Dock Connector cable that came with your iOS device.
Restart the computer.
Launch iTunes and try restoring.
If the issue persists and you are using virtualization software to run another operating system alongside your OS, quit that software; it may create virtual USB ports that may cause the issue.
4. Check security settings (Deactivate or uninstall Antivirus Software)
Out-of-date or incorrectly configured security software, firewalls, and network router settings or firmware cause most restore issues. Security software and utilities manufactured by companies other than Apple (third-party) must be kept up to date and properly configured to effectively protect your computer while still allowing an update or restore to proceed. Follow the steps in Troubleshooting security software issues to identify, update, and configure third-party security software. If you are using a router to connect to the Internet, either bypass the router and connect directly to the Internet source or go to the manufacturer’s website and determine if there is an update or if there are steps to configure security settings. If you have updated iTunes since checking for security software updates, your security software is likely to be out of date.
If you have installed software to perform unauthorized modifications to your iOS device, that software may have redirected connections to the update server (gs.apple.com) within the Hosts file. You may receive an error 1004, 1013, 3194, or “This device is not eligible for the requested build” in this circumstance. First, uninstall the unauthorized modification software. Then follow the steps to edit the Hosts file and allow iTunes to communicate with the update server in the Blocked by configuration (Mac OS X / Windows) > Rebuild network information > The hosts file may also be blocking the iTunes Store section of iTunes: Advanced iTunes Store troubleshooting.
After updating and configuring your security settings or modifying your hosts file, restart the computer and try to restore. You may need to temporarily uninstall third-party security software to restore your iOS device if the most recent version of that security software has not been updated to the latest iTunes release. After restoring and syncing your iOS device, reinstall and set up compatible security software.
If there were no system software updates available for your computer, you are running the latest version of iTunes, have no other USB devices attached, have no security software installed, and are directly connected to your ISP source, simply restarting the computer and the iOS device can clear up certain issues that could prevent you from restoring. After restarting the computer and iOS device, attempt to restore again.