Hi there,
So with the QSPI flash I know its a set BIT only kinda write thing so, Pages must be reset or cleared prior and as far as serial goes, sounds like either with usb the “while serial” is in the code or the buffers are getting over-run when NO serial device is present?
Without seeing the code these are just guesses. Use the Code tags </> above and paste in there the main.c others will comment as well.
HTH
GL 
PJ 
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