oh, so it's not immediate with all devices.
I suppose when you say "USB case" you talk about flashdrive. please, don't use "USB" for flashdrive, because EVERYTHING is USB (flashdrive, but also HDD, even mouse, keyboard !)
if it's flashdrive, then it also happen with low consumption devices.
it's strange that it can work longer if plugged before boot.
It happens when doing nothing? boot, and wait on system menu?
If it's really a power issue, it's strange too.
- why it takes some time to shutdown, the hdd is init right away and if the power is not enough it should be affected quickly.
- unless the power adaptor is decreasing the power little by little, but then it would also happen without plugged devices.
- why would it shutdown only with hdd or flashdrive plugged?
The power adapter has a kind of memory in it.
you can reset it by unplugging it fully (both sides) for few minutes/hours.
It happened to me once, the wii didn't boot at all (no red light), I unplugged it for a day. it's working great again