If Magnussen is so smart, why didn't he claim to have backups hidden somewhere, even if he didn't? I knew what Sherlock was thinking when he asked that question, and I'm not supposed to be in Magnussen's league. (Yeah, I've a genius-level IQ, but only just, while Magnussen is supposed to be up in the Sherlock zone. Even by real-life genius standards I'm less than mediocre.) Besides it's an obvious question, so Magnussen should have anticipated someone asking him that eventually anyway and prepared for it. I would've claimed to have seven or eight backups hidden around the planet, all guarded by people who would release the data if I disappeared or died violently or mysteriously. If I was so smart I could lie convincingly even to Sherlock, and it would take him weeks to be satisfied that I didn't have such backups. By which time, of course, I would have them, at least of the data regarding Mary.
That's the only way to blackmail somebody, after all, and I understand there is a record of that ploy being used in ancient times.