Since I installed the new USB hub on the home Mac Mini media server, I have used it pretty heavily with many rsyncs to the server, both to the Drobo and the other external HDDs. I've used it heavily doing a few things, and so far I would have expected several grey screen events, but so far have not had a single one.
So far, it looks as if the old USB hub was causing the problems. This is a great relief.
Now I can get on and use the Drobo the way it was meant to be used. I have two 3T drives and 2 500G drives in it, and that only gives me 3.78T storage in total. Needs to be expanded. I'll look at at adding two 4T drives soon.