On Thu, 14 Jan 2016 05:44:50 -0500, Micky wrote:
What wears out in an HDD? Is it only the tone arm that breaks? or
can the bearings the platter rides on break??? Good watches use
jewels, rubies, as bearings; and cheap watches use metal. What do
hard drives use?

Most of the causes is due to combination of heat and force.

IMO, the arm and bearings and are pretty solid but it's not impossible for
them to break - depending on the material and manufacturing quality.

https://en.wikipedia.org/wiki/Bearing_(mechanical)#Service_life

Does the spindle really ride on an air cushion?

From where did you have that thought?