It depends on the physical size and maybe speed of the hard drive. The 12v for the motor is the issue. Powering it from the card, you're routing it through the motherboard traces, connector and the card traces first. Lots of places to fail, or to add resistance and drop the voltage.
For example, I used an old 500MB 5.25" full height drive--- in an external box --- but I would never try powering that through a card. ;-)
