Flash memory indeed has limited write cycles. However, by now it is unlikely that you'll encounter this within the normal lifetime of such a card. Usually, this is in the order of 100,000 write cycles today and SD cards include circuitry to manage wear-leveling, that is, spread out writes over the storage media evenly to avoid "hot spots"—pages that are written too frequently and therefore failing early.

Information stored on the card is safe even in magnetic fields because the information is not stored magnetically (contrary to hard drives or floppy disks).

As for storage conditions ... you shouldn't store them in mud, water, lava, or other harmful conditions. You probably also shouldn't put them on railways and let trains drive over them. Apart from that, not paying particular attention to where I store my cards I haven't had any adverse effects so far. In practice, I'd think whatever doesn't physically damage the card won't harm the data on it.

https://superuser.com/questions/17350/whats-the-life-expectancy-of-an-sd-card#answer-17377

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