Why would my external USB3 drive (a Drobo) keep getting disconnected?
Are you using the USB cable that came with the Drobo? I know I got a bad one with my Drobo 5D that Drobo had to replace. I also recall some discussion a few years back about issues with Drobos sometimes disconnecting when the host went to sleep. Is your Mini set to never sleeps do not to sleep the connected drives?

I’m using a Thunderbolt connection with my 2012 Mini so my only USB/Drobo experience is a 5D with Windows 10.