During the course of decommissioning an old, failed 3.5 inch disk through application of a sledgehammer, I managed to pop-off the riveted cover plate to reveal the mechanical innards.
This was a single-platter 250GB Seagate Barracuda. Platter was aluminium rather than glass.
And here is the reading head, cause of those data-destroying head crashes. It amazes me that for four decades we trusted our data to these spinning masses of metal.