Hi and welcome to JustAnswer - I'm John.Can you please provide the error message Time Machine reported in its entirety?
Unable to complete backup. An error occurred while copying files to the backup volume.
What version of OS X are you using? To obtain this information, click on theApple icon at top left, then choose About This Mac from the menu. The numbers tothe right of Version is what I'm looking for.
Please go to Finder->Applications->Utilities and launch Disk Utility. Choose your WD external hard drive from the list on the left. Go ahead and Erase using Mac OS Journaled (Extended) and after it completes, go to Time Machine, make sure it knows to use that hard drive, then try the operation again and please let me know the results. I'll be back shortly.