Drobo spontaneously starts "data protection"

My Drobo just went from four green drive lights to “data protection” mode, where they’re blinking alternate green and red. I didn’t replace a drive and no drive is flagged as having failed. What are the conditions which can bring this about?

This is a first-generation Drobo, 8Tb total drive space.

Later: The data protection phase only lasted about ten minutes. Now it’s got four green drive lights again. What the heck is with this? If I had to guess, I’d say that one drive hacked up a bad sector in the middle of a large-ish Drobo file system “region”, and the Drobo re-laid out the whole region. Am I anywhere near right?

Really, really makes me wish Drobo Dashboard could report detailed S.M.A.R.T. status from all drives…

if a drive takes too long to respond drobo will assume its been removed/failed

if that same drive is relatively quickly re-added rather than doing a full re-layout and recreating 2tB of data drobo can just do a quick check that everything is ok - which is what sounds like happened

send your log files to DRI and they will tell you which dive had the issue and why