This sounds fairly familiar, after the latest firmware update I rebooted my drobo gen 3 and it showed 4 red lights and reported that to many drives had been removed.
A firmware downgrade did not help.
I eventually reset the drobo and it showed 2 drives as failed. I removed these drives, checked them in my PC which found no errors, I reused 1 in the drobo which then initially showed it as good and a while later as healed.
I have reluctantly come to the conclusion that the drobo is not fit for purpose as the data is less secure on the drobo than the individual drives as there are numerous reports of minor problems on 1 or 2 drives leading to a total data loss on a drobo.
I only now use mine for data backups which I can afford to loose.