Which power USB hub uses if it has external power supply as well

"USB ports can supply up to 500mA per port for powered ports, and 100mA per port for unpowered.
Also, the hubs usually say they draw 100mA of power, that is why you usually see four port unpowered hubs (100mA for the hub, and 100mA x 4 ports = 500mA which the powered port can supply).

Not all devices require 100mA, for example, I had a mouse that take 20mA. I assume that as long as the power the 7 port hub is less than 500mA, you will be fine. If it is more, Windows is smart enough not to power on the devices to protect the USB port.

External hard drives are usually powered by themselves, so I assume their power requirements are low. If you are in Windows you can see the power requirements in the Device Manager, under Universal Serial Bus controllers. There will be a list of all the hubs. Looking at the properties of the hub there is a Power tab. It will list all the devices attached to the hub and their power requirements. Many devices will list 100mA because that is the maximum an unpowered hub will supply.

I hope this helps explain why the devices may still work, and put your mind at ease that the computer is probably smart enough to protect itself.

(The only thing about using external drives is some drive can be self-powered or bus powered. Some of those drives will spin slower if they are bus powered, and will spin faster when self powered because they can pull more current. I've only seen this once so I don't know how common this is, the external drive had a laptop drive inside. This may be something to consider if you have drives of that type.)"

"I'm not quite sure what you're asking, as the question can be parsed many ways, but here is my stab at it...

If your trying to unplug the hub from the PC and therefore all the accessories plugged into the hub, you should eject them first. Not doing so can potentially corrupt those hard drives.
If you just want to unplug the power from the hub but still leave the hub connected to the PC, the hub will draw it's power (which it feeds to the accessories plugged into itself) from the PC. As this is usually not enough many accessories will cease to work. It is quite possible that in effect this is functionally similar to #1, where one physically unplugs the accessories before ejecting them. This, as mentioned, can corrupt them.
If your concern is that the motherboard will be damaged due to the need to supply all that power, you can rest assured on that score. The motherboard will supply what the spec is (or however the manufacturer implemented it), asking for more won't provide anymore, and correspondingly there is no risk of burning out that port."