detralt :
Am i missing something here? did i order the wrong part? my hdd is wd blue 1tb, the best selling on amazon. what am i missing here?
You bought the correct drive.
"3.5 inch" is simply nomenclature.
This stems from the old days, when a 3.5" floppy case and mount was standardized. The actual spinning media inside was 3.5". Obviously, the case was a bit larger. ~4"
Enter hard drives. These were sized to fit in the same case mounts and screw holes, for compatibility with existing cases.
20 years later, it is still a standard.