Re-formatting a external Western Digital My Book drive My new Western Digital My Book external drive will not erase to re format for my iMac. 

That is to say it starts but then goes no where. No green in the bar. What's up?
With the limited information you have presented, formatting drives with Mac OS 10.0 (2001) to the newest version 10.13 (Dec 2017) has always been the same. I recommend you try mounting any partitions first. If there is data on the drive but your Mac device cannot read it even after attempting to manually mount the drive partitions, then I recommend plugging the drive into a computer with support for the file system being used on those partitions. Ext 1-2-3-4 are all read and writeable by the majority of Linux operating systems, FAT32-ExFAT-NTFS-ReFHS are all read and writeable by most Microsoft Windows operating systems. 

Mounting


Open the Disk Utility app located in /EXAMPLE-HDD/Applications/Utilities/Disk Utility.app or by pressing CMD+SpaceBar and typing "Disk Utility" or by manually clicking on the Spotlight Icon in the top right (magnifying glass icon) and typing "Disk Utility", or by opening your LaunchPad and locating Disk Utility. 
Expand the Western Digital My Book drive on the left side and right click on the partition you wish to mount, select Mount. (Or Unmount and then try remounting it).



First Aid

Some logical volumes (partitions (file systems)) become corrupt during plug and play or electrostatic phenomena. However if there's enough data left the operating system can correct these errors.


Open Disk Utility (yes again, if you want other recovery software you may choose to use Disk Warrior or a deleted data recovery application). 
Located your Western Digital My Book and right click it.
Select Run First Aid... 
Click Run or press Enter.
Consider doing the same for any partitions you can access.



Formatting


Open Disk Utility, an application you can find by typing "Disk Utility" after pressing CMD+SpaceBar (or br clicking on your spotlight icon, a magnifying glass in the top right of the screen). 
Right click on the drive or any of the partitions and select erase. Choose the appropriate file system format: If you only plan to use to drive on Mac Computers then select one of the Apple Journaling options, if you plan to read and write the drive using anything other than Mac OS devices, I suggest ExFAT. 
Name the drive and erase it. 



Please Note that there are GUI differences between Mac OS 10.0-10.10 and 10.11-Current (10.13 as of writing this).