Very interesting; indeed I remember having tried with and without the USB, but I don’t remember exactly what happened.
(coming back hours after to correct this once checked) YES, the trouble came from the USB charging being connected. Without USB connected, the flash trick does allow to switch off!
Thank you!!
Sure, but the fact is, I bought it for him (indeed I now have two new FP2s 
) and I do want to prepare everything before giving it (he lives quite afar from my home : I want to be able to get access, update etc. and above all start from an efficient base)
Now, your link to 18.04.1 sounds perfect -I’ll do this as soon as I’m back home 
Thank you very much!
H.