adafruit/Adafruit_SPIFlash#1 has source and UF2 versions of an SPI flash erase program for all current Express boards. Accept this pull request if appropriate, and make the .uf2's available somewhere, along with documentation for why and how to use them.

Brief doc:

If your host computer starts complaining that your CIRCUITPY drive is corrupted, you can erase it completely. When CircuitPython restarts it will create a fresh empty CIRCUITPY filesystem.

Warning: this will erase all your files in CIRCUITPY.

1. Double-click the reset button.
2. Copy the appropriate .uf2 to the xxxBOOT drive.
3. The on-board NeoPixel will turn blue, indicating the erase has started.
4. After about 15 seconds, the NexoPixel will start flashing green. If it flashes red, the erase failed.
5. Double-click again and load the appropriate CircuitPython .uf2