Drobo customer support is trying but is coming up empty. Now I’m trying Data Rescue 3 to read data off the Drobo. DiskWarrior can’t see the Drobo because it isn’t mounted, but Data Rescue 3 can.
In fact, DiskWarrior also doesn’t see the Drobo when it’s run stand-alone off their DVD! I tried this on my wife’s iMac. OS X sees the disk (but does not mount it), Disk Utility lists it and allows me to try a Verify Disk and Repair Disk, but DiskWarrior doesn’t see it. I’ll try their tech support next.
Seek Nuance 
Thank you! I am also struggling with a non-mounting Drobo. Thanks to your suggestion, I am trying my Data Rescue Two disk. It’s worth a shot!