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Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
@Gordon
@Stev ahh - yes, that's your problem. The USB bootloader is also in flash, and to make sure you can't easily brick your Espruino, it won't overwrite itself.
That means that if you write an image to Espruino using the USB bootloader, the first 10kB (on Espruino) or 16kB (on Pico) probably won't be right. If you check the results of the verify you're fine if everything above there matches.