It looks like i'm 2 years late to the conversation but i'm experiencing the same issue with a 2020 Macbook Air Running Catalina. My external hard drive is plugged into a usb adapter to thunderport. Shortly after the HD icon shows up on the desktop it quickly disappears and i get the error that the device was not ejected properly(just like the rest of the folks on this thread). Does anyone have a solution? Does apple read this? If so why haven't they chimed in with a resolution? I've tried all the quick fix steps suggested throughout the thread and no luck. Anyone? Please?