Originally Posted by DuckieHo
Got somewhat of an answer...
At one time all PC hard disks spun at 3,600 RPM; in fact, for the first 10 years of the PC's existence, that was all there was. One reason for this is that their designs were based on the old designs of large, pre-PC hard disks that used AC motors, and standard North American AC power is 60 cycles per second (60 Hz): 3,600 RPM.
I seems like most are based off the 3600RPM.
10000 1 2/3
12000 2 1/3
15000 3 1/6
This is just somewhat of an answer and I'll show you why. In the USA ONLY, not all the North America the standard AC is 60Hz, Canada and Mexico are on 50Hz. The only other country I know of that runs on 60Hz is Brasil, maybe there are others, but their size is insignificant.
As a starting point 3600 came from here, the 60Hz in the US. 1Hz=1RPs, so 60Hz=60RPs which means that 60Hz=60*60rpm=3600rpm. This is true ONLY for synchron motors that run on AC, inside the PC this does not have any meaning whatsoever because all is alimented in continuous DC.
Still, you have a point. In the industry there is what I would call "inertia". People tend to stick with what they know and in this case it's likely that since there were small electric motors and bearings that were tested and working @3600rpm they decided to use them and not waist time reinventing the wheel. As a matter of fact between the AC Synchron motors and the DC motors the differences are quite small.
The next steps were quite logical if you ask me: 5400 rpm (+50%) and 7200 rpm (+100%). Other steps like 4500 or 5200 were illogical, thus abandoned. I frankly don't remember seeing many HDDs at these speeds.
A nice thing worth mentioning is the 4200rpm, the speed of the notebook HDDs. Why 4200 and not 4500? I would bet it has something to do with the power needed. As I mentioned earlier, the rest of the world is on 50Hz, thus 3000rpm, while the US is on 60-3600rpm. We engineers (I'm one of them) know very well what are the issues when we want to convert the europeans/canadian engines to the US standard, to raise their rpm by 600. It is likely that when trying to add some more performance to the aging 3600rpm HDDs while keeping in mind they have to stick in very strict power envelope they decided to raise the speed by 600. Why? Because the engineers had a pretty good idea what is to change so that the 3600rpm motors work @4200 without once again reinventing the wheel.
The 7200rpm is quite logical, double the speed of 3600 to gain a max of performance. I remember buying in 1998 or 1999 a new Celly rig and it was a lot of fuss if should my friends and I buy 7200rpm HDDs. The first were made by Maxtor if I remember correctly. This speed has been chosen as the next logical step after the previous step of 50%, aka the 5400rpm. I don't think there was more to it than that. They had to pump up the rotational speed of the platters and it was logical to continue with another 50%lift.
You probably know that the enterprise HDDs were developed from day one as high performing HDDs no matter what are the costs. I have no explanation as why have they chosen 10000rpm over another speed grade. A hunch would lead me to believe that they simply went for the maximum economically available at that moment from the material pow. It had to be over 7200 obviously, but at that time I don't think anyone could risk in designing a 20K rpm HDD considering the material fatigue, vibrations, heat disposal, ventilation and all the other constraints. They started almost from scratch with these HDDs and they've chosen "the best" under the circumstances.
The next logical step in the enterprise sector was a 50% increase in speed, thus the 15K rpm HDDs appeared. I have some experience with the kind of engines that work at such a speed with next-to-no vibration. The designer has to use in a very tiny space an motor that spins at a huge speed, thus it needs quite a high induction per square millimeter. This induction creates a magnetic flow that MUST stay inside the motor, because if it so happens that it leaks it destroys all the data on the platters. We need to use high quality materials that are pretty expensive and I suppose the demand for this kind of HDDs in not high enough (dunno how much should it be) to force the prices on such materials down. My bet is that we'll wait a while before this kind of HDDs would reach consumer level pricing.
What a man can do for a rep+
Actually I was waiting since a while ago to talk to someone about this. I find that many say that "HDDs are so far behind the other components in the PC", but few wonder why this is happening. Hope this clears up some things.