Thanks for the response, and suggested leads.
But I'm not sure if any are applicable. I'm not using WiFi nor Bluetooth. In fact I'm using a slightly modified version of an established Arduino (rorstandard) to implement the functions of a roll-off-roof astronomical observatory. Part of the IndiLib system.
I haven't seen this dialog (probably because I'm neither using WiFi nor Bluetooth).
I searched the system log before my post as I wasn't very comfortable with reporting a non-saveable crash report. But there is nothing.
So thanks for the tips, but I'm afraid I'm still in the dark and, as you agree, total system crashes just should not happen except in in very unusual (and hard to trace) circumstances.
Thanks,
Richard
PS: terminal: it works most of the time, so, for example now, after a Mac restart and reprogramming the Arduino, it's happily playing its part in my (simulated) observatory and moving the roof as commanded.
PS2: due to the above, I don't think it's the USB cable.