After downgrading back to dashboard 2.8.5 there are no more mounting problems with my drobos any more, which i have unhappily seen often since my upgrade to dashboard 3.0.0! …and after any new start of the system after downgrade to dashboard 2.8.5 i can reliably see my drobos always again!
After all, my advice as a cautious and disappointed user:
Do not upgrade to dashboard 3.0.0,
at least with MacOS Sierra with Drobos using an usb-3-Port!