I haven't whined about my Drobo for a short while, so it's time for me to whine some more.
Since the last time, lots of things have happened. I upgraded the Mac Mini to El Capitan. I should note that the Mac Mini is performing really well now, and have experienced no problems, shut downs, reboots, freezes, any problems at all, since Apple replaced the motherboard.
The Drobo has had a firmware update, and the Drobo Dashboard has had an update which means that driver files on the Mac Mini have been updated.
So maybe the problem between the Mac Mini and the Drobo have been fixed? Previously, if I left the Drobo connected, after a day or two I would see that the Drobo had been improperly dismounted (but not by me) and I would have to shutdown and reboot to get access to it again.
So I mounted the Drobo and left it for a couple of days. It did not get dismounted. That's one step forward. Unfortunately, while it stayed mounted, it did not stay accessible. I could read from it, but could not write. It had mysteriously become read-only behind the scenes. I had to dismount it and remount it to get write access to it again. I didn't have to shutdown and reboot, so that's a good thing.
So the connection between the two remains unreliable. Doug has a similar setup at home but he's using Firewire to connect his Drobo. It works reliably (but probably means an older Drobo).
I will continue to leave the Drobo dismounted, mount it when I need to backup, and dismount it again when done. Not ideal, but at least I can still use it.
Doug suggested that maybe something frequently reading from it would keep it mounted correctly. He suggested I get BackBlaze working again and back up just one directory on the Drobo. That's a good idea, and I will try that this weekend.
My brother suggested I push the Drobo through a USB hub so that when it connects to the Mac it's been forced down to USB 2. I will also try that. Most of the complaints I read about the Drobos and Macs point to USB 3 as the culprit.