It was Solid. I powered down a few times and up again, same result.
Then I powered up, hot removed the drive and shut down. Powered on, hot inserted the drive, and the Drobo found it. It is now restoring… :). Fingers crossed.
Worrying though. I have a friend who was looking to buy a Drobo but I’ve had to tell him in his shoes, with the support available from Drobo (which doesn’t seem that good once your device is no longer sold by them), that I’d build a Windows machine with Storage Spaces, since he has a spare box and all the drives he needs anyway.