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

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.