I have a couple different portable external Hdd's. I have a kill-a-watt device which is a meter you plug into an outlet and into which you plug an electrical device to see what the device's power usage is. I'd like to see how much energy these hard drives are using while they spin. These drives have a single port that you plug into the computer. They are powered through the same USB connection which transfers data. Can I plug the drives directly into a USB "wall wart" power supply (such as the one that charges my phone) , which is plugged into the kill-a-watt, without harming the drive? Thanks!!