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I vaguely recall thinking the file was just some sort of serial number or stock tracking metadata when I deleted it. It might have instead failed for quite mundane reasons such as low quality manufacture. I don't know.

Exposing the firmware in this fashion would be a bad idea for all of these reasons. It is a good thing that SD card manufacturers don't do this any more, if they ever did.

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Firmware definitely would not be on the filesystem layer or even accessible on the block device.

You'd think yeah

It isn't. Before a chip has firmware loaded, it can't decode filesystems.

That doesn't at all mean that the firmware can't be exposed through the filesystem. Is that a bad idea? Almost certainly. Is it possible? Absolutely yes.

Even after a chip has firmware loaded, which controller can make logical sense of a filesystem?

According to SDXC specs, the controller looks for exFAT markers to relocate the table and bitmap to smaller AU units.

exfatprogs can pack bitmap with table for SDHC and lower: https://manpages.debian.org/experimental/exfatprogs/mkfs.exf...

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