Punched card/clay tablets, fired as pottery. Very durable, machine readable with 19th century techy (Holerith tabulator), useless as roofing tiles (because perforated/leaky) so unlikely to be stolen and repurposed.
But really, no digital tech will survive a civilizational collapse by much more than one average human lifespan. In general only religions and larger buildings and the odd royal family go the distance. Human cultural artefacts are terribly transient.
https://mas.to/@natureworks/112540561109017890
How about a massive vault with a mechanical lock. Hidden under a pyramid made of large, heavy composite ceramic blocks. The blocks have instructions for opening the vault, qirh primer, etched on their surfaces.
Put whatever you want in the vault. It's air-tight, with a nitrogen atmosphere. Books, mostly, but also lots of demonstration mechanisms. A wind-up battery.
Layers of increasingly sophisticated vaults, one inside the next. The entire complex duplicated all over the planet.
@brent @cstross Long Now already have some thoughts https://rosettaproject.org/?ref=longnow.org
@brent @cstross as does Memory Of Mankind, who distribute clay tablet maps to their vault https://www.bbc.com/future/article/20161018-the-worlds-knowledge-is-being-buried-in-a-salt-mine