Answer here, not documented in manual, or apparently generally known:
https://www.mobileread.com/forums/sho...d.php?t=195609
The device will only be recognised if the actual USB cable is connected while the device is powered on, and active.
Standby mode is not good enough, or just generally turned on. It has to be active.
Likewise, you must plug in the cable while the device is turned on, you cannot plug in the cable and then turn it on.