From Register Hardware:

External storage supplier La Cie has found a way to quieten its external drives by adding staggered notches to the trailing edges of the fan blades. How does that work?

This is my understanding of the airflow and acoustics involved. A typical drive or CPU-cooling fan has, La Cie says, seven blades. These blades are angled and, as they spin, they cut into the air and send it at higher pressure (pressure side) underneath the fan blade and at lower pressure (suction side) over the top of the fan blade. The two airflows meet and merge at the trailing edge of the fan and vortices form as the two flows' different pressure levels get equalised.

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