Problems w WD Discovery and Time Machine

I have installed WD Discovery software on my iMac (running High Sierra) in order to use the My Passport 4TB HD with Time Machine. The back up works fine for a day and then consistently sends the message "backup failed". At these times I've tried to eject the HD to see if that would get Time Machine to work but a message saying the drive is "in use" pops up. My only recourse is to shut down the computer, start up again and then the back up works for another day.

This is crazy. I suspect the WD software may be the problem. Anyone out there having similar problems, and better yet, anyone have a solution?

Thanks.