Errors 2000-2009 (from Apple support): If you experience this issue on a Mac, disconnect third-party devices, hubs, spare cables, displays, reset the SMC, and then try to restore. If you are using a Windows computer, remove all USB devices and spare cables other than your keyboard, mouse, and the device, restart the computer, and try to restore. If that does not resolve the issue, try the USB issue-resolution steps and articles listed for Error 1604. If the issue persists, it may be related to conflicting security software.
- The Mac OS X kernel extension "IOUSBFamily" that was bundled with late 2008 / early 2009 MacBooks, as well as the 10.5.6 update, has a bug where it fails to properly recognize an iDevice in DFU Mode. It can be resolved by updating to 10.5.7 (or later), or using a USB hub.
- Close limera1n.
iTunes can't connect because another program is using it or it's disconnected during the Apple Server check.
- Connection problem; change USB cable and check USB port.
- sn0wbreeze failed to restore IPSW; unplug and plug iDevice and try again
Problem connecting. Probably a hardware failure.
Somebody reported that reinstalling the USB drivers fixed the problem.
Replace the USB cable with a newer one and/or disconnect all other USB devices, then retry.