'Disc not ejected properly' repeated error messages, 2019 iMac, OS Mojave 10.14.6

I have a WD Elements 4TB external drive (correctly formatted) for photos etc. Recently the 'Disc not ejected properly' message keeps appearing. I have tried: swopping the Thunderbolt ports, checked the drive with Disc First Aid (all OK), used System Preferences to ensure the HD is not put to sleep and computer does not sleep when display is off - and nothing works. It's still happening. Any ideas please?