With 1,382,400TB of raw storage, assuming a maximum of 2 drives is active per device tray gives us 92,160TB of actual, raw storage at any one time. If we then assume that the ratio of original:copy data is 1:2 (for availability), we have 30,720TB of usable storage at any one time.
30PB is still an incredible figure, considering that there are potentially 15 multiples of this (totalling ~460PB) of disk storage from that all-disk datacentre.
I don't know how the data placement algorithms work, but it would seem most logical if the relative objects are collated and spread across one particular lot of drives, unless these drives are themselves broken down into logical groups which may be necessary, given the sheer number of drives that could be active at any one time (23,040 drives in total x 4TB = 92,160TB). Drive groups could then be spun up and spun down based on user activity, for instance, one individual flicking through photos in various albums they own or are tagged in.