What a lot of people in this thread are missing is that, up until very recently, you didn't need anything but good eyes and a decent light source to see data. You didn't need tape drives of a specific format, or floppy disc readers, or lasers; the mark 1 mod 0 eyeball and daylight were sufficient.
I understand that people are already having trouble with this; amusingly, this became a plot point in the TV series Flashpoint in one episode, where the key clue to unlocking the reason behind a crazed gunman's standoff was contained in eyewitness testimony from twenty-five years before... recorded on a Betamax cassette. Time is counting down, the bomb's ticking, and cops are trying to find a Beta-compatible VCR before it's too late... However the idea is even older; in Fredrick Pohl's Heechee novels human prospectors discover the ruins of an alien settlement in an asteroid, and they don't even recognise that the aliens left behind entire libraries because the "books" aren't shaped like anything a human would use and have no markings visible to the naked eye. (The books end up being sold as cheap souveniers to tourists because they're shiny and pretty.)
Printed material, on the other hand, is incredibly durable. Recently scientists discovered a lost mathematical treatise by Archimedes [http://www.sciencenews.org/view/generic/id/8974/title/A_Prayer_for_Archimedes], written 2200 years ago, despite the de-inking and re-use of the paper to make a prayer book fifteen-hundred years later. (Admittedly, this time they used some super-cool tech... but not to guess the format, only to clarify what could be seen by the naked eye.)
Fifteen-hundred years from now, what are future historians (or, if we're unlucky, archeologists) going to make of Blu-Ray? Will they have the specifications to even be able to read one, or recognise it as anything but a pretty-shiny bangle probably used for decoration?
As someone who took some archeology and a lot of history, it does concern me a bit.
-- Steve