pressing and holding the power button may damage the operating system but i don't think it does anything to the hard drive since the 5-second duration allows your hard drive's actuator arm to move out on top of the platters.
even a warm boot doesn't do physical damage since the hard drive is still operational. for me, ang main makaguba kay kung madiretso lang ug kapalong ang hard drive kay ang read/write head mohagsa sa platter.