While power supplied through a USB port is generally sufficient for one device (such as a mouse, keyboard, joystick or gaming device, portable external hard drive, etc.), splitting that limited power among multiple devices causes them to work erratically or not at all. Most USB connected devices also require electricity to function, and unless they have their own dedicated power such as a battery, that low-volt / low-amp electricity is supplied through the USB "plug". A Multi-port unit powered ONLY through the USB connection to the computer is limited to the power of that one USB line. Think of watering a lawn with lots of hoses and sprinklers attached to only one faucet; the water pressure out of any one sprinkler is very low because all the sprinklers and hoses are sharing one source and that source only has so much to give. A good multi-port USB hub has external power to evenly distribute sufficient power to all the ports in the hub. This is true whether the hub has only one device plugged in, or many devices. Avoid multi-port units that have no external power. You'll save yourself time and money!