Atsutane Dot Net

Lately i see my HDD cant hold up so many file anymore. I need more space to store my file. Thumb drive isnt a solution for me So i was thinking to buy external hdd around 40GB or 80GB. I see my fren external hdd, it was so small kinda the size of scientific calculator. But the problem is, when i go check the price list it has 2 kind of external hdd. One is notebook HDD and another one is desktop HDD. I only see the different between this 2 type is the size. Other then that i dont know

Well the only thing i know is both external HDD support USB. So i think it can play nice with desktop and notebook But still donno kinda scare wan buy. Later buy cant use T_T. Anyway 3.5″ is for big or long ?