While many issues fixing third party drives require additional third party utilities, or repair with Disk Utility, Western Digital is unique in that it has some software it installs in your Mac HD -> Library -> LaunchDaemons folder (don't confuse with Users -> yourname or System -> Library which are different), that you won't find with most other drives. This software may break with certain software updates, and it has been reported, that simply removing this file made the external drive visible again. That file is called:
You will need an administrative password to remove the file while the drive is not connected. This will allow the drive's software to recreate the file and work properly again under your new operating system update. If you run into this problem, do contact Western Digital, and let them know it is an issue with your particular system, so they can know an update needs to be made, or where to direct you to an update.