POSIX FS open() Fails

Edit: This may have failed because of a deviceOS/build version mismatch, testing now.

Original Post:
I’m new to using the onboard flash storage. Eventually I want to be able to log while offline and then dump files to serial or OTA.

My first attempt failed. I have trimmed the code to a simple example where the Argon’s onboard LED D7 should be lit if I get a valid file descriptor from open() but it doesn’t succeed:

#include <fcntl.h>
const pin_t LED = D7;

void setup() {
  pinMode(LED, OUTPUT);
  digitalWrite(LED, LOW);

void loop() {
  int fd = open("test.txt", O_RDWR | O_CREAT | O_TRUNC);
  if (fd != -1) {
    digitalWrite(LED, HIGH);

Help? Or does anyone have an example of working POSIX file system code?
I am using an Argon and have tried with DeviceOS 2.2.0-rc1, 2.1.0 and 3.1.0 with the same result.

This was due to a version mismatch. open() works as expected, tested in deviceOS 3.1.0