What firmware version is your My Cloud running?
The Seagate Backup Plus should work. I use a 5TB Seagate Backup Hub Plus (purchased from Costco) with my v4.x firmware version My Cloud for Safepoint backups, and it works just fine. Have done multiple Safepoint backups to it. Didn’t have to change a thing on Seagate Backup Plus Hub drive. One thing I have found with the Seagate Backup Hub Plus is to attach it to the My Cloud or computer and wait a few minutes, sometimes it takes a few minutes for the drive to be properly mounted (generally on my computer for some reason).
I would suggest unmounting, using the My Cloud Dashboard, any USB drives. Perform a 40 second System Restore (no user data is lost) then let the My Cloud unit boot and scan. Once finished scanning then try to attach a single USB external hard drive and see if it’s recognized.
Other suggestions, make sure to update your My Cloud firmware to the latest version if it isn’t the latest version.
