Adafruit Feather M4 Express User Manual

If the bootloader couldn't start, you will get a red NeoPixel LED.
That could mean that your USB cable is no good, it isn't connected to a computer, or maybe the drivers could not
enumerate. Try a new USB cable first. Then try another port on your computer!
Once the bootloader is running, check your computer. You should see a USB Disk drive...
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Adafruit Feather M4 Express Specifications

General

Brand	Adafruit
Model	Feather M4 Express
Category	Microcontrollers
Language	English