Not that it's any less serious, but this all sounds like one or more software issues. Did you happen to notice if the new Drobo was running an updated software rev compared to your original?

Within IT circles, something this important/serious needs to be bulletproof. There's really little room for error, especially when it comes to integrity of the base filesystem. If it can't build out a new disk it should pop a warning and fail safe, allowing people without as much foresight as you to go and immediately make a backup.

Now imagine software issues in life-supporting apparatus. It's a dangerous world in the Adobe/Apple/Microsoft generation where "good enough" is usually what's good enough.

The Drobo has been around long enough that one would think any problems such as the one you've noticed would have been found n-times over and corrected by now. If it's something new, it would sure be nice to find out what the issue was. Do you know if the Drobo logs any of its rebuild process to internal flash or somewhere else where DataRobotics can look for clues to try and address this? I'm not a fan of the "send out a replacement and keep your fingers crossed it works this time" policy.