I don't know anything really about computer security, but I've read a few books about it. One thing I learned -- or thought I'd learned -- a long time ago was that thumb drives were a big threat but easily defeated: Just epoxy the USB ports.
I brought this up with a computer security expert some 20 years ago, and he told me, more or less, "We don't do that any more. We have internal methods that are better."
Seemed wrongheaded to me then, but what do I know? It's like keeping deadly weapons locked up where 4-year-olds cannot get at them. Seems a simple enough solution to me.
So how did Bradley Manning smuggle out his secrets? Thumb drive.
And Snowden? Thumb drive again, according to the Los Angeles Times.
My son-in-law has a security clearance, and at his facility (which I visited on family day last year), you cannot bring in a cellphone, or a camera (I was allowed to use a camera on the grounds outside).
This is in Hawaii, not far from the NSA offices where Snowden worked. Apparently the Air Force takes computer security more seriously than Booz Allen does.