I have had similar problems with Drobos. As long as there is no important data on the drives, what I found best to do is to only install one drive in the first slot. If you can see the Drobo in the dashboard and it doesn’t tell you to add more drives so it can create an array. I have found that doing a factory reset using the dashboard has always solved these types of problems for me.
A friend of mine brought a used Drobo on Ebay and had a problem like yours, he did the factory reset and the problem was solved.