Nope, that is the best part of these units. They know where the data is and where it needs to go. Also has a battery backup for memory so the power could have even gone out and you would be ok. I did simulated failures on mine when I got the gen 1 and the gen 3 just to record the behavior. Do you have dual disk protection activated? That is the only reason I could think of as to why it went to sleep on a rebuild after pulling the USB cord.
In some cases the rebuild does not immediately start, I would wait a little longer - should be no more than 5-20 minutes at the most. Also be aware that Drobo uses the largest size drive for redundancy.