I looked incredulously at your dimensions of the 3.5 inch HDD, only to search for the correct ones and found the exact same specs on Wikipedia, then realizing that's probably where you got yours. Looking at hard drive manufacturers' sites, I see that that is in fact the correct dimensions.
What does the 3.5 inch figure represent, then? The size of the plates?
The 3.5 inch figure represents the floppy, remember that? :)
A "3.5in HDD" is a hard disk that fits into the bay originally designed to hold a drive for 3.5" floppy disks.
I thought it wass due to the fact that the platters are 3.5", just as the 2.5" hd has 2.5" size platters.
But no, looked it up, 3.5" have platter sizes of 3.74" apparently, 5.25" had 5.12" platters. Which makes sense when you look at a hdd with the top off.
What does the 3.5 inch figure represent, then? The size of the plates?