When plugging in a new Android device to your computer for debugging, you might see this when executing adb devices:
List of devices attached
???????????? no permissions
This shows that 2 devices are attached, but only 1 of the two are working / recognised. To solve this you can try killing and restarting the adb server. To do this on linux, change to your android-sdk-linux/platform-tools directory in a terminal shell, then type:
Now try adb devices again and hopefully you see your device ok.