I have a few systems with a similar hardware configuration - several internal hard drives (up to 10) each with its own drive letter, plus at least one external Drobo disk array. My conundrum has always been - include the Drobo in the pool, or leave it out? I use duplication on all my folders...
Drobo's are great, they're fault tolerant using either single or dual parity; however when coupled with a DrivePool box, adding the Drobo to a pool that has all its contents duplicated would be a waste of drive space that's already protected by duplication.
So it would be great to be able to