Error 20, 21, 23, 26, 28, 29, 34, 37
These errors typically occur when security software interferes with the restore and update process. FollowTroubleshooting security software issues to resolve this issue. In rare cases, these errors may be a hardware issue. If the errors persist on another computer, the device may need service.
Error 1600, 1601, 1602: Try the following steps to resolve this error:
Ensure proper USB isolation troubleshooting has been performed, and test with a known-good cable.
Follow the steps listed for Error 1604.
This error may be resolved by disabling, deactivating, or uninstalling third-party security, antivirus, and firewall software. See steps in this article for details on troubleshooting security software.
Error 1604:
This error is often related to USB timing. Try changing USB ports, uninstalling and reinstalling USB ports, and other available USB troubleshooting steps (troubleshooting USB connections, device not recognized properly, computer won't recognize a FireWire or USB device). If you are using a dock, bypass it and connect directly to the white Apple USB dock connector cable. If the issue persists on a known-good computer, the device may need service.