Cannot run on USB-connected Android device

ionic run android --device does not work, ending with

ERROR running one or more of the platforms: Failed to deploy to device, no devices found.
You may not have the required environment or OS to run this project

This happens with the Android File Transfer app working fine with USB-connected Nexus 7 (running Android 6). However adb devices -l does not list the device.

Genymotion emulation of app works fine at another time (and shows up in the adb list).

Host is OS X 10. Help much appreciated!

BTW, the Testing your app doc page instruction to use ionic run android (without --device option used above) seems to not even look for the connected device, ending with

No target specified, deploying to emulator
No emulator specified, defaulting to AVD_for_Nexus_4_by_Google
Waiting for emulator...
emulator: ERROR: This AVD's configuration is missing a kernel file!!
emulator: ERROR: ANDROID_SDK_ROOT is undefined