More likely they need it to collect everything, including all of our calls and emails. So far, no one has been able to disprove such a theory that that is the more likely need for such a monster storage facility and we are supposed to be complete idiots apparently and think metadata (text records) require more than a few dozen servers at best and needs this to contain that text data.

That puts me on their top 10 cational sepurrity threats.

Squeeee!!!

... the metadata is from American citizens (and others) who have cell phones active on US networks. The NSA also collects signals intelligence - including communications content and high res areal photography - from all over the world. This foreign data is their core, and most important, mission.

In the case of diplomatic and internal foreign government communications, they would probably keep it around for a long, long time. Most likely because if they spot recurring patterns in past encrypted data it would give them the chance to decode more recent data. And, if they crack a more recent coded transmission, they could go back and decode previous communications. This historical information might be useful to an analyst as context for current situations.

The only way to top that-

The drought gets so bad they cant keep up their massive water usage, and the servers melt into slag.

as a drone?

And from all I can tell it was perfectly legal - from the story it sounds like they stayed at a proper altitude and I'm assuming they wouldn't be stupid enough to fly an airship without an appropriately rated pilot. SO.... unless the NSA manages to get a Temporary Flight Restriction slapped over their little bit of airspace there's really nothing they can do about it.