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Supersonic Disk Drive Is A Myth

Over the past few weeks, I've heard folks promoting solid-state storage solutions and repeating the myth that spinning disks haven't gotten faster than 15K RPM because it would mean that the airflow over the heads would become supersonic. While it's a good story, and the thought of little tiny sonic booms inside a disk drive is amusing, there's very little truth behind it.

For anyone who remembers his or her high school math, it's pretty simple to calculate the linear velocity of the outer edge of a spinning disk. Here's the formula, where LV is the linear velocity and RV is the disk's rotational velocity:


Of course, since nominal disk diameters are normally given in inches and rotational velocities in RPM, the formula above will give its results in inches per minute. To convert to miles per hour, divide by 63360 (the number of inches in a mile) and multiply by 60, which I assume you remember is the number of minutes in an hour.

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