a USB port can provide 5V, 5.25V maximum, & no less than 4.75V (basically 5 V±5%) and each USB port on an unpowered hub or directly out of the mobo may output upto 1A but usually it only provides 500mA, also a USB device may request more power, so if a device (HDD) requires more than 500mA it is normal to connect it to more than 1 USB port to enable the device to receive more power or it could request more power of upto 1A however if you use an externally powered USB hub, each port on a powered hub can provide upto 6A you can also attach 127 devices to a USB root hub (including the actual hubs but not the root hub), but you have to watch the power distribution or only use powered USB hubs, the way you attach hubs is: 1 root hub connector plugs directly into an externally powered hub which in turn can be attached to "say" 4 externally powered USB hubs & then each port in the 4 powered hubs can then be connected to another 4 powered USB hubs however a maximum of five powered hubs can be used, now if you do you maths and don't forget each hub is connected to the previous hub, now if you count all the hubs and connected devices you will arrive at the maximum number of devices which as you remember is 127 Source
