my portable HD has 2 USB cables, one for the computer and one for power... why?
Because it needs more power than the standard 5V the USB normally gives.
In order to work they must be connected to your computer, and to a power source. Would your car run if the engine were not connected to it?
many devices get power from usb. however the new hard disks, required higher current/power to run and it cannot be provided through single usb cable. hence you have to use two cables.
The small ones don't. The big ones do. They require the use of a power adapter (cable) to work properly. It cannot be powered by a USB cable alone.
3.5 drives need a power supply, the motor that drives the platters is 12 volt and the USB only supplies 5volt
