I can't see them using optical based on their price structure (compared to bluray media costs per GB). And I doubt they own a manufacturing facility to crank o

I can't see them using optical based on their price structure (compared to bluray media costs per GB). And I doubt they own a manufacturing facility to crank out super-secret multi-terabyte-sized optical disks. However it fits right in with the cost of LTO tapes. Quick back of the envelop: Their web site says 1$ per TB per month starting cost. That is $12/TB per year. A 12 TB LTO-8 tape costs around $70 retail, which puts it at $5.60 per TB. So the cost of the media used to store customer data is paid back in 6 months, or slightly more if they are using parity tapes. That would be my guess, the customer data hits disk first, then gets enough parity and split among enough data centers to hit their reliability target.