Immutable SD Cards

Date: 2019-01-25

Today, I had a strange behavior of an micro SD card that I've found around here. And with this blogpost, I will document this behavior as I had a hard time to search for this phenomenon , as it is confused by the "I physically switched by SD card to read-only mode, help!" problem in forums.

The phenomenon manifests as an immuteable SD card. The operating system thinks it is a fully working read-write block medium and the card reports no error for a written block. However, the block is silently dropped. And since the OS thinks that the write was successful, the modified block is placed in the buffer cache. Therefore, it looks like filesystem operations just work fine. However, when I unplugged the card and replugged it, the card snapped to its original state.

After wondering about this phenomenon on Mastodon, I got a lot of answers that this "magic SD card reset" is a common failure mode for SD cards. They switch into a (sometimes) silent read-only mode. As people have sugested, this happens for example if the maximal number of write cycles is reached and no erasure blocks are left.

PS: For this blogpost I tried to use as many googlable synonyms for the problem, as I had really a hard time to find a conciese description of the silent SD card failure.