I just bought two 7 port usb hubs: LogiLink UA0124. Just to run into a strange situation.
I did some servicing to my PC today and disconnected the main power cord from the PSU. Only to find out that all my usb devices(connected to other usb ports on the PC) are still powered up!
At first I thought that I somehow accidentally managed to make a perpetuum mobile!!!
When my euphoria run out I figured out that the hubs are giving a constant 5 volts to the PC port (backfeeding), even when I turn off the switch on the usb hub!
I also noticed that the processor fan has a small movement like twitching every 5 seconds…
So there must be some weird current running through my motherboard.
What should I do? This does not seem like some no name cheap product. So I wonder why is it designed in this way. Is it OK when the PC is powered up? I mean does the default voltage of the PC usb port counteract with this one and everything is fine? In that case I should only watch out when turning off the PC? Or is this something serious that could damage my other components?
UPDATE
As per request in the comments, I'm adding few pictures here. And many more on this LINK (LogiLink 7 port USB HUB dismantled)
Best Answer
I eventually ended up cutting the +5V(red) line going from the hub to the PC. The only drawback is that the hub obviously needs to be connected to its own power supply to work.