That was my issue initially. The Passport series of drives like to fall asleep on their own. It is part of the power saving tech WD decided to put in their firmware. I went and bought a WD Essentials 2GB drive and beyond a slight USB connector issue it has worked flawlessly.
Just my experience.
This thread may help, Gets a bit technical at times but it does provide some user’s experiences with various drives - https://community.tablotv.com/t/tested-hard-drives/130