Why HDD rotate at 7200 rpm?

I was wondering why hard drives rotate at these specific speed (5400,7200, ...) and not other values.

Solution 1:

A/C motors in many parts of the world commonly rotate at 3600 RPM because that's 60 Hz. 7200 rpm is obviously twice that, and 5400 rpm 1.5x.

I don't know the real reason, since HDD motors aren't driven from A/C, but it's likely IMHO that it's related to that. Like used to be possible with vinyl record decks, it's easy to check that something is running at the right speed by illuminating it with a strobe running at the required speed. If it's at the right speed (or a simple multiple thereof) then a mark on the motor will appear stationary.

Solution 2:

Like many things in computing, the reason is historical. The design of early PC hard drives was based on earlier, large, mainframe hard drives which were powered by AC.

That is according to PCGuide.