Good point about 3D, I forgot about it. Makes sense; I remember looking at how DNA is packed within a cell in "The Machinery of Life" and thinking just how ridiculously dense it is. But with such packing, you can mostly forget about random access; it's more of a tape storage. Which makes me think - if all we need is dense sequential storage of bits, then we should be able to do better than DNA by going with some simpler substances that can be stacked in layers or wrapped around some core. It's probably gonna be more complicated than nano-sized ball of bit-carrying yarn, but we don't need to replicate all the structural features of DNA.