Is an external hard disk with a metallic chasing better for heat dissipation than a plastic one?

Solution 1:

Metal generally conducts heat better (often much better) than plastic and will keep temperatures lower than a plastic enclosure, which can trap heat as it acts as an insulator.

I use a Rosewill RX-358-S, a metal enclosure with cooling fan, with a 1TB Seagate Barracuda 7200.12 and I haven't had a problem with it so far. Temperatures are kept under control at about 40-45 °C during the summer and the fan can lower temperatures by about 7-10 degrees more.

Solution 2:

I'd agree with you on that - the fact that the metal case is warm tells you that heat has been drawn from somewhere (ie: the disk). Barring fault conditions, it's a good sign that the case is warm because it means that it's acting as a heatsink for the disk.

Solution 3:

It depends on the specific drive and the quality of the design. A good design will dissipate the heat, a poor design might not. The material used for the case will figure into the heat dissipation scheme, but using metal vs plastic won't "fix" a bad design.

If one drive feels warmer than another, it may be because it's generating more heat in the first place.