real spinning disk
Yes, it's a 2.5" ElectroMechanical Hard Disk Drive spinning at 5400 rpm.
When one month ago I exhaustively tested the usb-bulk, I used progressive LBA addresses to read write read-back blocks (this is also what the program "badblocks" does), but if the problem is related to high current peak when heads move around, that means I need to run tests with random LBA addressing, which is the case of what the kernel does with the virtual memory, otherwise progressive LBA addresses correspond to progressive slow head movements, therefore less current peak.
Mumble
but it somehow makes sense, I mean the virtual memory I observed during the kernel stress-test moved the heads like a deranged dog on speed (I heard the sound the heads made, kind of "tic, tic, tic, tac, tac, tic"), and probably at some point this may require an high peak of current that the on-board LDO is unable to provide.
Umm, so a problem with the power on-board module, probably a tanker-capacitor or something similar
I will investigate and also try with a Solid state disk (flash-SSD or sdram-disk). But first I want to re-test everything on a virtual machine to make sure it's not a kernel bug.
(oh, here I assume the rootfs cannot crash the kernel, but it's a strong conjecture)
Thanks guys! Great advice!