"I still have the original platter which I retrieved from the rubbish bin. The head crash had gone through the oxide right down to the base metal"
I've seen one that went through the magnetic layer, and all the way though the aluminium base in a section (Yep, you could see right through). Somehow, it was still balanced enough to be spinning.
What stumped us, was not that the drive failed in this manner, but that it was in this failed state for long enough to gouge the platter without anyone noticing.