The amusing thing about the Bekenstein bound is that it's per millimetre squared, not per millimetre cubed. The amount of information you can pack into a given

The amusing thing about the Bekenstein bound is that it's per millimetre squared, not per millimetre cubed. The amount of information you can pack into a given region of space scales like length^2, not length^3. For the sorts of scales human beings are interested in, of course, the Bekenstein bound is a long long way from being relevant, and what matters for us in principle is volume rather than area. (There are practical difficulties in using "substantially" 3-dimensional regions for storage, but they have nothing to do with the danger of the storage device turning into a black hole.)