Flash isn’t a good long-term storage option. It relies on an electrical charge to store the data, and will discharge over time. It’s literally physically storing electrons, but those electrons are constantly trying to escape. Good flash may last 7-10 years without being plugged in, but the standard off-the-shelf stuff will be dead much quicker.

Tape or M-DISC are the gold standards, though both are more expensive than flash. Tape is by far the single most resilient method; it can even be reconstructed if it is physically damaged. But it’s also the least convenient and most expensive. M-DISC is a nice middle ground. It’s essentially just a burned disc, but made with materials that won’t rot over time like standard burned discs will. So storage is as simple as storing regular discs. Though if you’re truly trying to apocalypse-proof it, you’d probably want to consider bunkering/burying them somewhere to protect from physical destruction.