With Microsoft’s entry into the field, the use of DNA for data storage is an increasingly serious area of research. DNA is effectively a base-4 data medium, it’s extremely compact, and it contains its own copying mechanism.
DNA has actually been used to store data; in 2012 researchers at Harvard wrote a book into a DNA molecule and read it back. It’s still much more expensive than competing technologies, though; a recent estimate says it costs $12,000 to write a megabyte and $200 to read it back. The article didn’t specify the scale; surely the cost per megabyte would go down rapidly with the amount of data stored in one molecule.
Don’t expect a disk drive in a molecule. DNA isn’t a random-access medium. Rather, it would be used to archive a huge amount of information and later read it back in bulk. A wild idea would be to store information in a human ovum so it would be passed through generations, making it literal ancestral memory. Now there’s real Mad File Format Science for you!