In the first Star Wars film (“A New Hope”), Leia gives their only copy of the Death Star plans to R2-D2. Then later on, Darth Vader is informed that the plans are not on the computers of the Tantive IV, the ship that she was captured on. This leads him to correctly infer that the plans were somehow sent off-ship, probably through the escape pod without life.
Now, in the Legends background, the plans are transmitted to the Tantive IV through the ship radio, which means they likely had to be stored on the ship’s computer at one point. In fact, I doubt that the Rebels would only have a copy of the plans on the disk that Leia gave to R2-D2, because such a disk would be a quite obvious method of transporting the plans. Why not give a copy, and make the Empire believe that the plans are still on the ship?
Since canon might not suffice, answers that involve Legends are equally valid for me.