Just bought a spanking new iMac! Connected my 3 Drobos (Drobo 5D3, Drobo S, Drobo FS) and the only one that Drobo Dashboard recognizes is the Drobo FS.
The other two mount, but Drobo Dashboard does not detect them. You can see them in the screenshot.
I have rebooted several times and disconnected - reconnected the undetected Drobos.
Both of the missing Drobos are connected via USB.