Just a note to let you know not to install the latest firmware 4.2.1 on your Drobo 5D if you are still using an older MacOS.
I have my 5D connected to a Mac mini (2011) that cannot be upgraded beyond High Sierra, and after upgrading the Drobo to the newer version the Thunderbolt interface stopped working properly. I could still “see” the Drobo in Dashboard, I could mount the volume, but couldn’t write any data to it, despite it being mounted read/write correctly. The USB interface worked normally. Very very weird.
Upon downgrade to the previous firmware version (3.5.4) the Thunderbolt interface started working normally again.
Hope it helps someone as it took me a while to figure out.