Have a look at the physics for a moment:
First, lets examine what kinds of forces are acting:
- critical vertical movement
- moderate horizontal movement
- combined creates torsional as well as diagonal movement
Now let's examine what the movement looks like in a typical "bump" environment. For test purposes, our point of observation will be earth (that is, we are observing from a fixed point without any motion)
as your car hits the bump (while moving at a fixed speed) .. here is the motion of the drive:
position on the y axis indicates height .. we will determine the position to start at time 0 (which is moments before the test subject will hit the half-sine bump. Notice that opposed to typical convention, a negative change in height is represented by the (positive) portion of teh Y axis
time on the x axis indicates an arbitrary time interval of < 1 s
If we examine (the crude) graph, you will notice that the test subject (harddrive) will initially move down to either the height of the bump .. or to the tensile threshold of the springs.
Furthering our observations, we notice that the motion completes a sinusoidal motion .. which creates a large problem for us.
Imagine now that at the furthest extent of the motion (at the crests) the hard drive has stopped accelerating and has stored a considerable amount of potential energy in the springs ... this will create a very large instantaneous acceleration in the opposite direction (the direction of the net force) ..
If we put some numbers in the whole scenario, we would realize that the inertia of the harddrive upon instantaneous acceleration either up or down is quite large .. and this presents a problem with the drive's internal mechanics. This goes back to the concept that objects at rest (or in motion) tend to resist change (ie. motion in another direction) .. thus causing stress on the parts ...
I realize that this is both long winded and a fairly crude explanation .. though I hope it helps all the same.