hi alf, maybe the drive that initially failed, had some rubber feet that got stuck on the bottom. i think some samsung spinpoint drives had that issue but if you lift it up slightly as you press on the spring release clip that might help to remove it.
with what you mentioned happen, it might be worth waiting a bit to see what the drobo does, just in case the blinking is related to the failed one being put in again, though can you also have a look in dashboard for a couple of things when you get a moment?
firstly, does dashboard seem to show the same status or light, as what the physical drobo is showing you?
(can you also try exiting dashboard, and relaunching it)
secondly, in dashboard, can you click on each of the drive bays (or to use the dropdown for more info) to see if you can bring up any label messages about each drive, such as Warning or Healed etc?
one thing you could try (after leaving it at least overnight in case the status changes), could be to try using dashboard to shutdown the drobo, and then after about 5 minutes, to power up the drobo again, as a power cycle might help it recognise the status of the drives… however, if your drobo is currently accessible, then it is probably worth copying your data somewhere first, or at least the most important data, just to play safe, especially if you do not have a backup, beforehand if you can.