I have the 4 bay.. but was reading a Drobo review the other day..(can't find it now) and he was reviewing the 5 bay. He decided to test that functionality and pulled a drive then popped it back in... then another and popped it back in.. then he pulled a drive completely and then another completely. He said that not only did it not falter but that all data was still accessible.

I would not want to try a 2 drive failure on a 4 bay though.....