Does anyone have specific information about the protection device
at the power supply inputs of hard disks?

I assume that it's a zener diode or something similar in action
such as an IC that acts like a precision zener diode. It's not
difficult to see how such a device would provide protection
against spikes, over-voltage and reverse voltage by shorting the
power rail to ground and thereby triggering PSU shutdown.

Devices in the BUX C*** series seem to be widely used, but I have
not been able to find a datasheet or other detailed info. Can
anybody shed some light on the matter?