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When you say "3.5-inch hard drive," most people think of desktop PCs. But that presumption is changing as 2.5-inch hard drives find their way into desktop PCs and servers.

The trend first took hold with servers and enterprise hard drives, as 2.5-inch drives achieved high rotations per minute--10,000 or 15,000 rpm--and better performance while generating less heat than 3.5-inch drives.

Now, consumer PCs are moving to 2.5-inch drives as well. A push toward smaller, design- and energy-conscious systems underlies manufacturers' inclusion of 2.5-inch drives in desktops. Mainstream desktop manufacturers use laptop components to build the compact PCs of the future, so it makes sense that some compact PCs have 2.5-inch hard drives inside. Even power desktops can get into the 2.5-inch game with Western Digital's VelociRaptor hard drive (reviewed above), which zips at 10,000 rpm. Capacity is no longer an issue: 250GB to 320GB 2.5-inch drives are commonplace, and 500GB drives have begun shipping. But 2.5-inch drives won't take over on the desktop: Since 3.5-inch drives offer a better price/capacity ratio, 3.5-inch drives will continue to prevail.