Even more important question...
Why the hell did the original, unknown, user put classified NATO data on a memory stick in the first place? Jesus-tap-dancing-Christ, didn't anyone involved with setting up what must be a fairly complex NATO defense network ever stop to think about allowing users to make copies on removable media?
I mean, there are literally thousands of different ways to make secure dumb terminals, encrypted transfers, multi-token authentication, etc, etc, etc. Does NATO not have a single competent BOFH?
Hell, do they accept unsolicited resumes for high ranking positions? I could do better. Or, at least fake it better, which is basically the same thing.