Did a clean install of Yosemite (10.10.2) but even though the Drobo appears in the Finder and the shares can be mounted, the ‘Select Backup Disk’ dialog of Time Machine Setup does not see it. Is there a setting somewhere that prevents this?
Can you provide some more information? Which model Drobo do you have? I’m only familiar with the 5N, which has to have Time Machine support specifically enabled via the Dashboard on the share you wish to use for Time Machine backups.