USB drive mode is supported through the SingleFileDrive class which allows the Pico to emulate a FAT-formatted USB stick while preserving the onboard LittleFS filesystem. A single file can be exported this way without needing to use FAT as the onboard filesystem (FAT is not appropriate for flash-based devices without complicated wear leveling because of the update frequency of the FAT tables).

This emulation is very simple and only allows for the reading of the single file, and deleting it.

Callbacks, Interrupt Safety, and File Operations

The SingleFileDrive library allows your application to get a callback when a PC attempts to mount or unmount the Pico as a drive. Your app can also get a callback if the user attempts to delete the file (but your sketch does not actually need to delete the file, it’s up to you).

Note that when the USB drive is mounted by a PC it is not safe for your main sketch to make changes to the LittleFS filesystem or the file being exported. So, normally, your onPlug callback will set a flag letting your application know not to touch the filesystem, with the onUnplug callback clearing this flag.

Also, because the USB port can be connected at any time, it is important to disable interrupts using noInterrupts() before writing to a file you will be exporting (and restoring them with interrupts() afterwards). It is also important to close() the file after each update, or the on-flash version the SingleFileDrive will attempt to export may not be up to date causing issues later on.

See the included DataLoggerUSB sketch for an example of working with these limitations.

Using SingleFileDrive

Implementing the drive requires including the header file, starting LittleFS, defining your callbacks, and telling the library what file to export. No polling or other calls are required outside of your setup(). (Note that the callback routines allow for a parameter to be passed to them, but in most cases this can be safely ignored.)

#include <LittleFS.h>
#include <SingleFileDrive.h>

void myPlugCB(uint32_t data) {
    // Tell my app not to write to flash, we're connected

void myUnplugCB(uint32_t data) {
    // I can start writing to flash again

void myDeleteDB(uint32_t data) {
    // Maybe LittleFS.remove("myfile.txt")?  or do nothing

void setup() {
    singleFileDrive.begin("littlefsfile.csv", "Data Recorder.csv");
    // ... rest of setup ...

void loop() {
    // Take some measurements, delay, etc.
    if (okay-to-write) {
        File f ="littlefsfile.csv", "a");
        f.printf("%d,%d,%d\n", data1, data2, data3);