I noticed that many of the external HD's don't report their HD spin rate & I would imagine since most are either 5400 or 5700 RPM.
Perhaps the USB throughput does not require a faster spinning (7200 RPM), since it will never utilize the full potential & be a waste of resources (power & heat/HD longevity)?
I also notice that some Externals come with power supply, while others powered by USB only. Which is better to get? Which will last longer in the long run? Additional plug for power is annoying, but is there a benefit?