400TB in 5hr???
So in 18000 sec (5hr×60min×60sec) they saved 409600 GB... which roughly is a transfer speed of 22.7GB/sec. I'm wondering what type of SSD raid they've got :-)
My guess is they have a cabinet / 42U worth of HDD arrays, by the numbers they need ~ 176Gbps write bandwidth which is totally possible even in half a cabinet.
400TB worth of SSDs @ 176Gbps write bandwidth is also very possible, would be ~ 4U expect but extremely expensive.