Hi

I have been trying to use MTP to access the built in SD card on a Teensy 3.6 from Ubuntu 18.04. Using the MTP and SDFat libraries, the example MTP_blinky compiles and uploads with no errors. The serial monitor prints "I should be commented out" and the built in LED flashes c.3 secs on, c.3 secs off. However, Ubuntu can't detect the device.

Code:
#include <MTP.h>

MTPStorage_SD storage;
MTPD          mtpd(&storage);


volatile int  status = 0;
volatile bool sdfound = 0;
volatile int  count = 1;

void rtc_seconds_isr() {
  if (count-- == 0) {
    digitalWrite(LED_BUILTIN, status);
    Serial.println("I should be commented out");
    status = !status;
    if (sdfound)
      count = 2;
    else
      count = 1;
  }
}

void setup() {
  Serial.begin(19200);
  pinMode(LED_BUILTIN, OUTPUT);

  RTC_IER |= 0x10;  // Enable seconds IRQ from RTC peripheral
  NVIC_ENABLE_IRQ(IRQ_RTC_SECOND); // Enable seconds IRS function in NVIC
}

void loop() {
  if (SD.begin()) {
    sdfound = true;
    mtpd.loop();
  }
  else {
    sdfound = false;
  }
}
The libraries can be found at:
https://github.com/yoonghm/MTP
https://github.com/greiman/SdFat

49-teensy.rules is saved in /etc/udev/rules.d/
"MTP Disk (Experimental)" is selected in Arduino IDE (1.8.7). Teensyduino 1.44.
Selected port is "/dev/ttyACM0 (Teensy 3.6) MTPDisk"

I have tried the following techniques to no avail:
https://forum.pjrc.com/threads/43050...r-Contribution
http://bernaerts.dyndns.org/linux/74...any-mtp-device
https://superuser.com/questions/1334...-18-04-upgrade

'lsusb' shows that the Teensy is available via USB on Bus 006:
Bus 007 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 006 Device 010: ID 16c0:04d1 Van Ooijen Technische Informatica
Bus 006 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 005 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 001 Device 002: ID 7392:a833 Edimax Technology Co., Ltd
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 004 Device 003: ID 046d:c52b Logitech, Inc. Unifying Receiver
Bus 004 Device 002: ID 046d:c52b Logitech, Inc. Unifying Receiver
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub

'mtp-detect' (mtp-tools) does not find it though:
libmtp version: 1.1.16
Listing raw device(s)
No raw devices found.

I have tried to connect using file manager, go-mtpfs, jmtpfs and gmtp but none work. I have also tried different USB ports. Does anyone have any suggestions please?

The serial monitor prints "I should be commented out" twice and then stops. The LED continues flashing though. Is this behaviour correct?

Thank you.
Dean.