I poked around on the Drobo support site until I found a firmware update that’s supposed to fix the Error 51 problems. Unfortunately, the instructions only apply to updating a Drobo that the Drobo management software recognizes.
I hadn’t heard from Drobo support this morning so I decided to try some things myself. I shut down the server, and Drobo1 didn’t go into standby. I pulled the USB connector, and it still didn’t go into standby. I power-cycled the Drobo. It came up with just one blue light in the capacity row, and no lights at all in the column down the right side that indicates the health of the disks. After it settled down, I powered on the server. The display didn’t change, and Drobo1 didn’t show up in the Drobo management console.
Maybe it is a hardware failure. I’ll wait some more for Drobo to get back to me.