Crystalline salt, NaCl, should be a good storage medium for data. Chlorine has two abundant natural isotopes with atomic weights 35 and 37. The atomic weight of sodium, Na, is 11 and so NaCl requires 47 Daltons per bit. It would seem that there is no other substantial amount of support needed to maintain information thus stored in salt. Other storage technologies may store more than one bit in an atom but then perhaps require many other atoms arranged to maintain the integrity of the atom with the data. I think that individual atomic weights have already been measured by observing the vibration frequency of a cantilever with the subject molecule on its tip.
I think that the science fiction story “Fire Upon the Deep” mentioned a space ship carrying salt to serve as one-time-pad for encrypted communications since quantum computing had made conventional crypto obsolete. I do not recall that they mentioned isotopes of chlorine as the storage technique.
Reading and writing the data would be a mundane technology given nanotechnology. Reading would be destructive, as so many other data storage technologies are. An exabyte would occupy 226 micrograms of salt which is a typical grain of salt.
The first commercial use of salt was to preserve meat. People still speak of ‘salting things away’. Salt mines are currently seen as stable places to keep conventional data storage media.
DNA stores 2 bits in 560 Daltons. This is a factor of 6 in favor of salt but we know how to read and write DNA now. Writing, however, costs approximately 0.1 $/bit in 2014. Duplicating DNA using PCR is extremely cheap.
Massive erasure coding of the data can overcome scattered losses of data without much extra overhead. I can imagine redundancy much less than 2 for storing data in DNA. 80 equal size globs of data can be coded into 100 globs each of the same size. If any arbitrary subset of 80 out of those 100 strands survive then the entire set of data remains available. Per strand ECC is probably called for to overcome SNPs.
Here is some interesting information bearing on such ideas.