I first used a 3.5-inch hard disk drive (HDD) 20+ years ago. And I took it for granted that 3.5" HDD should be 3.5" wide.
Until today I realized that 3.5" HDD are in fact 4" wide.
What is the reason of naming those 4" HDD as 3.5" HDD?
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According to the List of disk drive form factors article on Wikipedia, 3.5" HDDs have inherited their form factor from 3.5" floppy drives. The disk inside 3.5" floppies actually had a 3.5" diameter, hence the name.
Interestingly, the 2.5" drives are actually about 2¾", but there's no corresponding floppy size. I think the creator of this standard has decided to scale down both length and width by a factor of 70% and the same logic was applied to the format name, producing about 2.45" which looks nicer when rounded up to 2.5".
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