So here is the situation.
The Mac Mini is rock solid without the Drobo attached. If I attach the Drobo, it will be improperly ejected within a day, sometimes several times a day, and the Mac Mini has to be rebooted to get the Drobo back into service.
It gets improperly ejected in the middle of watching a film from the Drobo, or in the middle of a rsync backup to the Drobo.
There appears to be no fix for this problem. The Drobo is no use to me like this.
I got a new Linux installation in place that has HTFS+ drivers. I connected the Drobo, and Linux saw two drives and could mount them but see nothing. That was useless. When I connected the Drobo to the Mac again, the Mac reported problems. I had to run the Disk Utility and let it fix the filesystem on the Drobo for two hours before it worked again. I won't do that again.
So there we are. The Drobo is no use to me. All my music and video is on the Drobo and I have no reliable access to it. My backups are on the Drobo, but it won't stay up long enough for BackBlaze to back it up, and I can't keep it on the Mac reliably enough to keep my backups in sync.
I have one last experiment to make before I get rid of the Drobo.
The blame points to USB 3. My older Mac Mini does not have USB 2, but my older Mac Mini won't boot as the hard drive is dead. Likewise, my MacBook has USB 2 but it won't boot because the hard drive is dead. I shall buy two replacement hard drives for these two, and see if I can get the Drobo working reliably with USB 2. It won't be as fast, but I'll take reliable over fast any day.