Asklemmy

Best bet is long term optical discs or long term magnetic tape. USB keys are not good for long term storage. USB keys use NAND memory that is a series of floating gate metal oxide semiconductors (FGMOS). These operate by using Fowler-Nordheim tunneling, in where a charge is carried along a regular style fin field-effect transistor (FINFET) and a charge above the transistor's channel causes some electrons to quantum tunnel into floating gates that are isolated by oxides.

While these floating gates are sealed off from everything, so the charge should stay "indefinitely", quantum effects cause some of the electrons to "leak" out of the floating gate, causing a degradation of the stored signal. Typically there's a refresh circuit within the USB key's integrated circuit that takes care of that and USB data can last seemingly forever. However, that refresh circuit requires a small amount of power, which if you store the USB stick somewhere for years on end, will never get powered.

This is the reason why flash memory only assures data can be retained for about ten years without power. Eventually the electrons "trapped" in floating gate have enough time to tunnel out of the floating gate completely obliterating the signal. The tunnel events aren't many per second, but give enough time, and all of those events add up. Paired with the whole thing that USB sticks mostly no longer use binary logic levels. Most are now using something like four or eight logic levels. So instead of there just being on and off, there is 0V-0.7V = 00, 1V-1.7V = 01, 2V-2.7V = 10, 3V-3.7V = 11 logic levels. So a small amount of charge loss can create a different bit pattern.

One thing to look at for long term storage is something like M-DISC. The matter by which the burned data onto the optical media is made is via a process that takes about 10,000 years (estimated) to break down. However, the disc itself is in a polycarbonate thermoplastic that has an average breakdown of only about 1,000 years in extremely dry environments and about a tenth of that in your average sealed lock box environments.

Your average spinning disk hard drive can store information for some time, but the storage requirements are pretty intense and even then hard drives loose about 1% of the magnetic strength per year without power. And about 70 years is the max before the various magnetic bits that form the low level format of the disk have degraded without power to the point that the disk has too many bad sectors to be called usable. But outside of that, the biggest fault is mechanical failure. No matter how well you think you've stored a drive, it's never good enough and the spinny bits always fail from becoming too fragile from pervasive oxidation. Basically the drive will spin up only to tear itself apart as some weaken part of the armature flies into the spinning platters.

But USB sticks will only give you about a decade before the stored information fades away into the quantum ether.