changed this in my code to #define SERIAL_BUFFER_SIZE 256
I also am using a SAMD21 board (feather m0) and the HardwareSerial.h file under "/Users/userName/Library/Arduino15/packages/adafruit/hardware/samd/1.2.1/cores/arduino"
Transmitting, then we generally don't check in the same way and just tend to get away with it, unless we start exceeding the TX buffer that
Personally, I think it is better to deal with the problem by implementing some sort of check, such as availableForWrite, rather than hide the problem in larger buffers,
#define SERIAL_BUFFER_SIZE 64
This is the downside of making it easier to provide 'new' cores rather than derivative cores.