XCalibur – the microSD in the stone

I have seen similarly weird results with flaky USB flash keys. A lot of those don't do any kind of hardware error detection.

The behavior described here can be explained by block device cache: You write a file to a device and the device doesn't report an error, but doesn't actually write the data due to some hardware error. When you read the file back, the content will come from cache in your RAM, not the device itself. Unmounting and mounting the device flushes the cache. When you then read the file again, it will come from the device and have the original content again.

Some of the cheaper cards (and USB keys back in the day) sort of assume you'll use a single FAT[/32] partition and they understand the FAT and use it as poor man's TRIM information. They get super upset if you attempt anything that is not a single FAT[/32] partition. Some will actively forbid it by ignoring writes to the first sector of the disk and of the partition. Perhaps this is one of those cases?

EDIT: technically this is kosher as SD spec mandates FAT32 (just like SDXC spec mandates exFAT)

SD cards' firmware can be hacked, see http://www.bunniestudios.com/blog/?p=3554

Of course, my stupidest hackathon project makes it to HN. Great.

    Jordan messes up and rm -rf / instead of rm -rf /dev/sdb

slapstick classic

I feel like there's some preface here that I'm missing.

Sounds hasty. There are so many layers between you and your SD card it is hard to know sometimes which ones are broken.

Typically, if you can't use dd(1) to write to it, and the error dd(1) gives is not 'write protected media', then your card is essentially dead.

I had exactly the same issue (except formatting my system).

SD card was broken, and it placed itself into read-only state for data recovery. I could delete files, but those would reappear. Formating would work until restart etc. Write cache was responsible for this weird magic.

I got card replaced as part of warranty.

This is pretty cool, I am really liking it.