When running: adb reboot bootloader on your PC if you see this error

Error: insufficient permissions for device

Solution

  1. Log in as root.
  2. Go to folder /etc/udev/rules.d/”.
  3. Create file “51-android.rules”.

  4. Obtain vendor ID of the device

    • Connect phone to PC via USB cable

      Here we take the example of the Gigaset GS290 device

    • Obtain vendor ID by running lsusb in bash.

  5. This should result in a screen with a few lines of output one of which would look like this:

    Bus 004 Device 004: ID 0e8d:201c MediaTek Inc

    Here it means that the GigaSet vendor ID is 0e8d.

  6. Maintain line below in file “51-android.rules”

     SUBSYSTEM=="usb", ATTR{idVendor}=="0e8d", MODE="0666", GROUP="plugdev"

  7. Assign read permissions on the file & reboot

     sudo chmod a+r /etc/udev/rules.d/51-android.rules

    Alternatively you can reload udev and the adb daemon with these commands

     sudo udevadm control --reload-rules
 adb kill-server
 adb start-server