What leads me to believe this is that this function:
//Sends out struct in binary, with header, length info and checksum
void EasyTransferI2C::sendData(uint8_t i2c_address){
uint8_t CS = size;
_serial->beginTransmission(i2c_address);
#if ARDUINO >= 100
_serial->write(0x06);
_serial->write(0x85);
_serial->write(size);
has “beginTransmission()” call which is only done by the Master I think…