Hi,
I've performed tests suggested by Jonathan in my system.
1st test:
I have started the system, connected the FLASH disk to the USB 3.0 port
and suspended it by "echo disk > /sys/power/state".
Then I pressed the POWER button to switch the system on again, and it
has resumed!
Similarly, the system resumed after "Hibernate" triggered from the KDE
menu.
2nd test:
With "i915.modeset=0" my system started with FLASH disk connected to the
USB 3.0 port
I attach logs (dmesg) and acpidumps after both tests.