Backup Problems

Check to see if the external drive is full or that it has enough space to hold the entire backup of the My Cloud unit (or what ever is being backed up). And as a troubleshooting step, use another external hard drive as a backup target. The following WD Knowledge Base Article may have some suggestions (if you haven’t seen it already):

My Cloud: USB Copy, Synchronize and Incremental Backup Failure
https://support-en.wd.com/app/answers/detailweb/a_id/5078

There are a few similar past discussions, if you haven’t seen them already, that may have further ideas or suggestions.