Go to Control Panel / System / Harware / Device Manager /
slide down to the list at Universal Serial Bus Controllers, you will find a USB Mass Storage Device with a yellow triangle. This is your external Drive controller that failed to initialize and will always remain like this even after reboot... until you right click Uninstall and when done, just right click on any of the devices in the list, (or in the Actions menu above) select scan for hardware changes ....and voila! Your external USB Drive will be regognized and start functioning as it should.
This applies to WD passport USB external Drives as well