Normally, 2.5" external harddisk cable will have two USB head.
one is connected to a thick cable the other is to a thinner cable,
Thick one is for data and power (the one that you are suppose to
plug into your comp) the thin one is to provide extra power if the
required power is not enough.
Normally, when power is not enough, the LED on ur external harddisk
will keep flashing unless urs doesn't have one.