I wonder what material they used for the platter. I once took apart a 1.8" drive, and got a big surprise when the platter suddenly shattered. I was expecting aluminum, not glass/ceramic substrate.
It is easier to ensure that glass substrates are perfectly plane and without any surface defects than for substrates made of aluminum alloy.
In 3.5" disks the risk of shattering becomes too great, so aluminum alloy is preferred.
