Bad blocks. On the disk in the Drobo. I've written about it here.
I'll make a long story short: after the incident I lost all trust in my Drobo and I replaced it with a Synology DS1512+. It's a different beast, it reads the S.M.A.R.T data and it has already protected me from one disk outage, as I experimentally put the bad disk in it that Drobo wouldn't recognize as bad. I received a prompt warning about the issues with the disk and the Synology Diskstation simply took it out of the loop. That's how I envisioned this to work.
It also seems that I'm not the only one who has retired their Drobo(s), Scott Kelby also did. Also seems that if you're high profile enough, Drobo's CEO will take personal care of you. For me it was a bit of a different story, guess my talking about my Drobo issues in public wasn't too much of a PR disaster for them.
Anyway, I'm off the Drobo and on the Synology and I am very (!) happy with my decision.