Re: "Not an uncommon story", you mean.
I was not there but saw the effects of a rotating drum (preceded the spinning disks) that had come off its thimble and crashed through the cabinet, down a ramp, through a major door and across the hall.
I don't know the speed of this thing (attached to a GE-635) or its weight but I can imagine it was quite substantial.
As far as those possibly apocryphal stories about disks of death, we did used to dare each other to stand in front of a drive unit and press the emergency off button. This button was supposed to apply an immediate brake to the disks, probably making them useless but also possibly causing them to fly off at various angles. The height of this rotating mass of rusty bits was just below waist level. In those days most of us cared about the equipment down there.