'Positive' in this connection may have meant something different to you from what I intended, which was maintenance of a positive static pressure within the case, as opposed to the negative static pressure during air extraction by an exhaust fan. I envisaged that air be blown into the case by the fan, and therefore that it follow (more-or-less) the path intended for convected air over the power/sweep board and out through the top vents. How much dust ingress there might be would depend on the point of air input, and the presence or absence of a filter at the input. Having air flow through the FDD from heads to insertion slot could well be an improvement over the reverse direction of airflow, although that is at much slower flowrates during convective cooling.

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