Comment by mrb

The 60 Hz grid is why we have HDDs spinning at 7200 RPM. Because of 60 Hz, it was simpler to design AC electrical motors to spin at 3600 RPM or 7200 RPM (1x or 2x factor). When DC motors were made, for compatibility with AC motors, they often spun at the same speed (3600 or 7200 RPM) even though there was no electrical reason to do so. When HDD manufacturers selected DC motors for their drives, they picked 3600 RPM models as they were very common. Then, for performance reasons, in the 1990s, 7200 became more common than 3600 eventually.