As to the Drobo, I am not sure you can read SMART data of a USB attached Drobo from your PC.
So, unless Drobo firmware reads and exposes SMART data it its gui - it doesn't -, there's no way to know.

I think.

Back to my failed 3TB drive: I could read it from my PC, but I only see 750GB, and it hangs repeatedly. I'd say this is indeed fa failed drive.