[Robert A Cook PE]

But a CD is a simple plastic disk, only modestly balanced.

A hard is machined aluminimm, much srtonger, thicker, and balanced. Surface is smoother too -> less vibration.

[Calvin Locke]

But a CD is a simple plastic disk,

A bit more, but yes, I doubt it was designed for the desired speeds (xfer rates) nowadays.

BTW, last time I disassembled a hard drive, probably a 3.2GB, the disks were less than half the thickness of a CD, iirc.

[Flying Circus]

HDD disks are either Aluminum or Glass substrate. I think the glass works better in high RPMs applications because it distorts less from the centripetal forces.

You are right about the need for balence. Especially in the high speed drives that get stacked in big arrays in servers. Put 20 drives side by side and run them at full speed and you get a lot of vibration.