Glass substrate
The disc a hard drive's magnetic film is deposited on — glass instead of aluminium gives flatter, stiffer platters that tolerate higher speeds and lower flying heights.
The platter in a hard drive can be an aluminium-magnesium alloy or glass (glass-ceramic). Glass, brought into use by IBM and HOYA in the late 1990s for 2.5-inch drives, is flatter, stiffer and more thermally stable. It tolerates higher spin speeds, lower flying heights and therefore greater areal density with less flutter and warp under heat.
It is also more shock-resistant against head slap in laptops. Aluminium is cheaper and dominates 3.5-inch desktop platters; glass rules the mobile and high-RPM space. The magnetic layer, underlayers and overcoat are sputtered on identically either way — same coercivity, different carrier.