A hard disk drive Or optical disk drive can be thought of as a device that sends and receives signals.
Eg:- Consider a case where a CD player is connected to a amplifier. We can power the two devices using a single source or two power sources. (In this case the power sources are isolated)
The same applies to the motherboard and hard disk drive scenario.
Hey check out the sata data specification. No power is given to the hard disk throught the data cable.http://en.wikipedia.org/wiki/Serial_ATA