The HDD in your mirror is much larger than it appears
"To reach 20TB by 2020, the 500GB/platter drives will have to increase areal density 44 times in six years. It isn't going to happen."
Huh? to reach 20TB by 2020, areal density would need to double a little more than 5 times in the next 6 years. (.5TB / 1TB / 2TB / 4TB / 8TB /16TB), not 44 times. All right, this is not gonna happen either.
More likely that we'll 4 or 5 platter (or maybe 6 platter) 3.5" HDD's at 40TB by 2020, which only requires 2 generations of capacity doubling, as vendors put the HAMR down.
Larry @ NetApp