The problem is not the existence of 5TB hard drive. The problem is how do you carry a 5TB HD in and out from a NSA office. The places are highly guarded, and they'll monitor for everything that comes in and out. You might have some luck sneaking in small devices - like pretending you have a commercial CD, but it's actually a writeable CD - but sneaking in and out hard drives it does not seem possible. And if you do it with other medias (CDs, DVDs, zip drive) it's a LOT of external media to carry around.