My Book Thunderbolt Duo copy to My Book Duo failures

I work off a WD Thunderbolt Duo EHD, and copy to a My Book Duo EHD so I always have a backup. I use SUper Duper copy software. When I schedule a copy I get error notifications on the WD drive; cannot be accessed, raid issues etc. However if I just do ‘copy now’ it runs without a problem. The SUper Duper support says the EHD has hardware issue, however I can work off any of the EHJDs without problems, and disk utility says everything is OK.
Any ideas what might be going wrong?
Any ideas greatly appreciated.