Ever double-check an assumption, only to find it false, and feel like the rug just got yanked out from under your feet?
Go measure the width of a so-called 3.5" hard drive. Yeah, go ahead. It's 4" wide. Now go measure the width of a so-called 2.5" hard drive. Uh-huh... it's 2.75" wide. I just (re-?)discovered that and felt just as cheated as when I found out that the nominal sizes of dimensional lumber are complete B.S.!
I knew this once upon a time, but I guess I forgot... 5.25" & 3.5" refer to the fact that it fits into the bay where you would have been able to fit a floppy drive accommodating the corresponding-size floppy disk. I can't find any derivation for why we call a 2.75" drive a 2.5" drive, since there's no such thing as a 2.5" floppy[1]! Perhaps the internal platter is 2.5" wide?
(See: http://en.wikipedia.org/wiki/Hard_disk_drive#Form_factors)
-Adam
[1] OK, yes, there a 2.5" floppy did exist briefly, but it was never any sort of standard and I've never seen one in person AFAIK. FYI, the Sony micro-floppy format was not 2.5", it was 90mm (closer to 2", anyway).