well, the way im seeing things, under normal cirumstances the SDR protection protects you from 1 drive failure.
so in the case of a drive failing, id imagine 2 main ways for this.
1a) where the drive fails at the point of the drobo being switched off (and noticed when the drobo is next switched onn)
1b) where the drive fails at the point of the drobo being switched on (which is also noticed when the drobo is switched on)
2) where the drobo is in use, with working drives, but then a drive fails. (which is noticed during running use of the drobo)
(there might be a few variations inbetween stages of failing, or during a rebuild etc but i think of those 2 mainly)
one way to simulate a drive failure, is to boot up all the way, (discharge any static from yourself etc) and to then eject a drive. after several seconds id imagine it going into rebuild mode. (there have been some reports that if the same drive is pushed back in again, that the array recognises it as being part of the disk pack, but it might just blank it and then re-assimilate it into the pack, via a rebuild.
however, as your drobo does not seem to be booting up all the way (at least from dashboards point of view), and if your shares are not mounting and no data is available, then im thinking maybe a removal of that 3tb drive while power is off, and then a bootup, might get it past its stuck point. (however, if your drobo didnt have much free space, it might not rebuild with sdr again). If it didnt rebuild, then the drive that we took out, should still be part of the disk pack (as i believe a rebuild re-defines the pack) but we’d have to see what happens before trying the other 3tb to avoid getting the “too many drives removed message”.
Please dont Hot plug the drives as for yoru last question because im pretty sure that you will wipe your data that way
i think lets wait to see what some other users think, as its always good to get a second opinion, and in case i made a mistake as havent had enough coffee at this hour yet
hang in there buddy and hopefully a solution can be found for you