Serial.read has garbled data after a Serial.write
I've finally found that when the MCU  to which Arduino is talking to was initialized, it would send spurious data to the Serial - I guess it would start first with a lower baud rate, and then switch to the programmed baud rate of 115200.

To fix this, I had to flush the Arduino serial RX before sending any data. So:

byte streamReadResponse;
Serial1.begin(115200);
Serial1.setTimeout(9000);
while (Serial1.read() &gt;= 0)
           ; // do nothing
Serial1.print(cmd);
Serial1.print('\r');
Serial1.flush();  //wait for all the data to be sent to the serial
streamReadResponse = Serial1.readBytesUntil('&gt;', data, dataLength);