I finally replaced my USB charger by the official PSU, and I do confirm it solved my issue. Thanks.

I am just a little disappointed RPi cannot be used any more with any USB charger as it was possible with RPi 1 or 2.

I will order one, hoping this will solve my problem.
Thanks.

(But I still not understand why voltage shortage occur when there is high I/O, and not when there is high CPU load)

From the start, I assumed that RPi were to be used with any phone charger. And it was OK with my RPi 1B & 2B... That was great, cause it was cheap to reuse all that old phone chargers. ;) Of course, I made sure to use some branded stuff. Can you please explain (or give me a link explaining) the ...

I use a RPi3B+ for remote backup. It's powered with a 5.1V / 2A Asus power supply. 2 USB hard disks, with their own power supplies, are connected to the RPi. At my home, it was OK. When I installed the RPi at my mom's house, it rebooted after some hours, even without any load. I replaced the default...