After a ton of research and experimenting, I finally found the solution to my USB connected Drobo 2 being seen in Dashboard, mounted, visible in Control Panel but not assigned a drive letter.
I am using Windows 8.1 but this also applies to Windows 7. My Drobo is formatted with HFS+, for which I cannot recall why I did that if I was using Windows.
Regardless, all I had to do was load an HFS+ for Windows driver. Installed the Paragon HFS+ driver and after a reboot came back to life! Drive letter was assigned and I am back in business.
You would think this is in the Knowledge Base for Drobo but it isn’t.