Pressing and holding the power button will not cause damage.

Sudden removal of power from a hard disk will not cause damage.

The only damage that could be caused is the the head will crash into the disk surface and scratch it. Under normal circumstance the head is kept from hitting the disk by what is known as the Bernoulli Effect. This causes the spinning disk to create a high pressure area on its surface which the disk head cannot get through and it effectively floats on a cushion of air. This cushion of air is about five microns thick which is about half the thickness of a human hair.

When power is removed the disk stops spinning, but it doesn't stop immediately. There is enough time for the head to be moved to the outside of the disk in a blank area called the landing zone. When the disk has slowed down sufficiently the head will settle on this landing zone without causing any damage.

In the early days when hard disks where relatively new the drive head was controlled by a stepper motor. This created the problem that if power was suddenly remove the head would crash into the hard disk at the place it was damaging data. During a normal shut down procedure the hard was "parked" over the landing zone before power was removed. A sudden loss of power and this could not happen as a stepper motor needs power to do anything.

This led to the introduction of the voice coil to control the heads. It is called a voice coil because it behaves in a similar way to the coil in a loudspeaker which is just a solenoid. When power is removed the solenoid reverts to its rest position which moves the head over the landing zone. It was now possible to move the head without the need for power and one big problem was solved.

Pressing and holding the power button for four seconds by passes the shutdown procedure which is
under software control, and simply removes power from the power supply. Pretty much the same as pulling the plug except that is saves groveling around the back of the computer.

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