CD's aren't as robust, and nobody has a floppy drive any more (also, unless you want all your info in .txt files, you can fit fuck all on them)
That said, cheap 500mb / 1gig SD cards might tempt people. They might be out of somebody's camera, and that person might have taken lewds with the camera. But "direct" SD readers aren't as common as USB ports, so they might just put it in their own camera to see if there's pics on it - thus not seeing pdf's / docs, and only seeing text-heavy images on a tiny camera screen. Not finding lewds, they'll just format the card and keep it.