I’ve read a lot of forums and they are all saying the same across the board. The new updates in firmware is causing the drives not to mount.
[b]Software ver# 2.8.4 
Hardware ver# 3.5.2 [7.179.86336]
Operating system: OS X El Capitan Version 10.11.6
All I did was a firmware update. Nothing else was changed. THE ISSUE IS THE NEW FIRMWARE!!
Please Don’t reply with try this or that. You will be wasting my time. All I want is the SOLUTION if you have one. Everything that was posted by other users with trying different cables, or OS etc, has been done. Same results. Drobo you have enough evidence to confirm you have an issue. I have submitted a ticket. Waiting to hear a resolution. Possibly a way to go back to previous firmware? (Doubt that)
Can’t believe we rely on a firmware update to improve our equipment, not to be STUCK, NOT ABLE TO WORK. How else would we know this could have been avoided. (Please take this thought to your next corporate meeting)