Thanks tannewt
This clarify why "myfile( ...128 bytes...)" (obviously slightly larger than 128 bytes) fails.
In the meantime I found a menu in uPyCraft to force the board to be considered as a microbit which enforce a smaller transfer chunk (64 bit). Forcing this mode seems to make the file transfer reliable.
I was about to report this tonight.
May be we could add a short comment in https://circuitpython.readthedocs.io/en/latest/ports/atmel-samd/README.html (Connecting ... serial...) ...


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