- 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.