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Comment by sgtnoodle | Hacker News Reader

Removing an SD card from its socket during a write operation can actually brick the card enough to no longer work at the block level. Perhaps it's brand dependent, but I'm talking about a well known name brand. It's rare, but I know of a ziplock bag of bricked SD cards that shows it's not too rare.

I suspect it occurs when a card loses power while in the middle of shifting data around for wear leveling purposes. Presumably, some internal data structure gets borked in a way that the card's embedded MCU firmware didn't anticipate and can't recover from.

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It doesn't even have to be during a write operation. Nand needs to be periodically refreshed after a read, so the controller can be shuffling data even on a read-only mounted card. I've read speculation that most consumer cards store their controller firmware on the same nand that is used as the general storage, and thus are prone to corrupting their own firmware if something goes wrong while shuffling data.

Power loss is definitely a threat to an SD card in a Pi. But I've seen many cards go bad while the system is still running, so it's not the whole story.

Weirdly, I've had a number of cards that entered a state where they would be inaccessible under Linux or Mac OS. But they could easily be recovered by a re-format under Windows. No idea why. Perhaps the manufacturers test a lot more thoroughly with Windows?

That's a good hypothesis re: the MCU firmware on the NAND. If it's true that all NAND needs to be periodically refreshed, then it could be as simple as losing power while one of the firmware sectors is getting refreshed. That seems like an obscure enough scenario that there's dragons lurking around it.

I like to imagine in my head that the MCU in an SD card can detect its voltage dropping quickly enough to get into a safe state most of the time. With all the recommended capacitance around the socket, it might actually have a few milliseconds to work with after external power loss. When you pull the card out of a socket, though, the MCU stops working before it has time to do anything about it.

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