This question was inspired by https://superuser.com/questions/374386/how-to-store-and-preserve-lots-of-data. There have been other similar questions, but none with the same criteria. This is two

Short answer

It’s impossible to guarantee a long timeframe because of entropy (also called death!). Digital data decay and dies, just like any other thing in the universe. But it can be slowed down.

There’s currently no fail-proof and scientifically proven way to guarantee 30+ years of cold data archival. Some projects are aiming to do that, like the Rosetta Disks project of the Long Now museum, although they are still very costly and with a low data density (about 50 MB).

In the meantime, you can use scientifically proven resilient optical mediums for cold storage like Blu-ray Discs HTL type like Panasonic’s, or archival grade DVD+R like Verbatim Gold Archival, and keep them in air-tight boxes in a soft spot (avoid high temperature) and out of the light.

Also be REDUNDANT: Make multiple copies of your data (at least 4), and compute hashes to check regularly that everything is alright, and every few years you should rewrite your data on new disks. Also, use a lot of error correcting codes, they will allow you to repair your corrupted data!