Drobo adds, effectively, a chunking layer. Suppose that all drives are integer multiples of 1 TB. Split each drive into 1TB volumes, and make mirrors such that no volume is mirrored to another volume on the same drive. You gain storage efficiency at the cost of performance and complexity.

In the scenario above you could get 8TB protected by mirroring, or use a Z1, Z2 or Z3 scheme.