Hi @lehippie,
Just tried out the sketch, instead of using a mic, I connected pin 3 (FS - frame select) to A6 (SD -serial data) to simulate I2S data. After a 20 minute test things are still going. How long does the condition take to occur?
In general, opening and closing the fie for each sample is going to have a big hit on performance, same applies to storing the value with a timestamp. This will result in dropped samples.
I'd suggest you explore the callback API calls, they are more advanced but for something performance oriented like this needed. There's a brief discussion in GitHub here: ICS43432 I2S sound recording and playback · Issue #2 · arduino-libraries/ArduinoSound · GitHub
Again, recording stereo audio data to an SD card is not something we explored in depth, because our tests showed it's not fast enough with the erase cycles. You could definitely look into storing mono data though.