USB WD Drive letter is not shown in My Computer

Does the Ex HD have a power cord?

That would have to be plugged in if it does.

Was the computer on, when you plugged in the USB cable?

If not, try that, the computer should recognize it when you plug in the USB and if you don't have the proper drivers, should give you the option to install them.

#1 Make sure the External USB drive is pluged in to the power outlet

#2 Make sure the usb cable is connected from the External drive to the back of the PC.

#3 Conform all drivers have been installed.

#4 Make sure in device managers there are no Red X or yellow question mark on USB devices

# 5 go in to the BIOS and confirm all USB devices are set to enabled.

# 6 make sure there's not a group policy enabled on the computer.

My WD external does this every so often. All I do (after many trials) is disconnect the power to that drive then reconnect. That works for me!