Had the Drobo for about two weeks now and I’ve bought a third drive for it, so I’m up to 3TB total storage and I’m at 1.4TB usable (1x 500GB, 1x 1TB, 1x 1.5TB), so it’s basically keeping a mirror copy of the data that spans three disks at this time. A mirror copy is giving me 50% of the usable space, which seems correct. So the algorithm is: Use the max space that you can span across multiple drives. If you only have two drives, your usable space is the same as your smallest drive. Three drives, is a more complicated algorithm, but improves the usable storage to almost 50% of the total storage, which is what we’d like to achieve with RAID1 also. I’m not sure that they use parity in the Drobo, so it’s more of a mirror situation that spans multiple drives.

Anyway, the system seems to be very stable and inserting drives can be done live without having to worry about any data availability. I could theoretically lose two drives with no data corruption.

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