#fatfs

I wanted to copy a file off the flight controller's SD card and get the same bytes back. Small files worked; anything past a kilobyte stalled or came back subtly wrong, non-deterministically. This is the hunt that followed — an isolation ladder that exonerated the SD card, a plausible theory I ruled out by measuring instead of guessing, and two bugs with the exact same shape: a synchronous primitive racing an asynchronous event that only collides under a preemptive scheduler. Eight fixes later, the round-trip is byte-perfect to 64 KB.