Windows 7 hung at Logging Off phase

This problem would arise when attempting to shut down the PC. Most times it would hang while displaying the message "Logging Off" or on occasion it would get past that and then hang while displaying the message "Shutting Down". Investigation showed that the cause was a well-known one, namely an external USB disk drive connected to the system. These are often the source of Windows shutdown hangs and the correct procedure is to "Safely Disconnect Hardware", then power-off the external disk drive before attempting a shutdown. While it is possible to write a script which performs the "Safely Disconnect Hardware" during log-off and/or shutdown, the power-off still needs to be done manually. If the external USB disk drive is not powered down, it can also cause the PC to hang during the boot sequence when the BIOS attempts to configure USB ports.