[citation][nom]jn77[/nom]Well the way you have to look at 3.5 inch drives compared to 2.5 inch drives is, if the density per platter on a 2.5 inch disc was applied to a disk 3.5 inches in diameter and the rotational speed was increased so the outer edge of the 3.5 inch disc was spinning enough, then per platter a 3.5 inch drive would way out perform a 2.5 inch drive by a long shot, and you are not waisting space for extra cases around all those additional 1.5inch drives reducing cost and putting in denser storage media in the same space.[/citation]
For high RPM, smaller disks can spin faster, so that's a moot point. Furthermore, some form factors can't fit 3.5" drives (IE many blade servers, like I said earlier) because there's simply not enough room. The smaller drives are also more energy efficient.