Why would my external USB3 drive (a Drobo) keep getting disconnected?

I have a mac mini (Mid 2011) running High Sierra (10.13.4) that I've been trying to use a home server. I'm not sure what the cause is, but my external USB drive (a 3rd-gen 4-drive Drobo) keeps disconnected after 16-48 hours. The Drobo is out of warranty and since June 2017 Drobo doesn't provide any community support so I haven't been able to figure out if the issue originates with the device. Aside from power cycling (which isn't the cause in my case) what might cause a USB drive to disconnect ?