[brandmeyer]

Turns out that some outdoor environmental dust is ferromagnetic.

A particular vertical-axis wind turbine project was destroyed by buildup of magnetic dust on the generator magnets.

I'm not saying that's what was killing your drives directly. Normally HDDs are fully sealed. But the amount of dust you mention is awfully suspicious. It might change your investigative perspective a bit when you consider that some small percentage of all that dust is ferromagnetic.