The Drobo has two 500 gig drives and two 3 terabyte drives in it. That gives me 3.57 terabyte total storage. I've been transferring my data to it, and have completely transferred the contents of two of my external USB drives to it. And there have been no more Mac Mini crashes, no more grey screens. I pulled the two USB drives out of service.
But that left the Drobo very full. The line of blue lights at the base of the Drobo went to 9, meaning my Drobo is over 90% full. I got a notification, and I got warnings that it was full. The first 500 gig drive now shows a yellow light, and the Drobo continues to recommend that I replace it with a drive of larger size.