One of my 1TB WD Green drives failed earlier in the week while I was away. The Drobo emailed me indicated the fault and that it was low on disk space. I got home today and bought a black WD 2TB drive this time for 204USD plus tax. I pulled out the 'failed' WD 1TB drive and popped in the new one. The drive was hard to pull out. I ended up using a metal pin under the drive to get it to come out, the springs in the Drobo don't work anymore for that tray.
So, that solves the immediate problem of getting the Drobo working again. It'll take probably 2 or 3 days now to relayout the data using the new drive. It's currently got 3 x 2TB drives and a 1.5TB drive giving 5TB of usable space.
The next problem is why is the Drobo full? It looks like my time machine is gone crazy again and is continually backing up the whole drive again. Last time, this was the spotlight metadata so I'll try that again but maybe thats a symptom of my USB drive failing also. I think I'll check the smart status of my 1TB USB drive and the failed drive this evening. This is a problem with USB drives or the Drobo, you cant easily see the SMART status. I put the drives in a PC desktop that I have for games to see them.