According to the Drobolator, a Drobo-S with the Dual Disk Redundancy has a LOWER capacity with 5 x 2TB drives than a Drobo with 4 x 2TB.
This seems counter-intuitive since one would expect the dual redundancy to cost only one more disk than single error correction.
I do hope it is a Drobolator bug, because if that would be true, that would basically destroy any economic advantage the Drobo may have over mirrored disks (which are faster too).
Furthermore it would make the migration of a full Drobo to a Drobo-S impossible if one wants to benefit from Dual Disk Redundancy, which is the big plus of Drobo-S.
Drobolator results :
Drobo w/ 4 x 2TB = 7.3TB = 5.5 data + 1.8 protection
Drobo-S w/ 5 x 2TB w/ Dual Disk Redundancy = 9.03TB = 4.53 data + 4.55 protection
Drobo-S w/ 5 x 2TB wo/ Dual Disk Redundancy = 9.03TB = 7.26 data + 1.83 protection