My Essential did NOT backup drive

My hard drive failed and I figured everything was backed up on my WD external drive.  Basically, ONLY the folders were backed up and none of the files.  I lost over 1000 photos.  Why did the external drive NOT backup everything.  How can I make it work correctly now.  Product was basically worthless to me; with only the folders backed up. 

What did you bsck up with? Did you try data recovery software? Recuva is a free one