Hi, I have a 500gb mac mini. I want to buy a good value usb external hard drive to use with time machine for a back up. I read somewhere that for time machine the external hdd should have more gb than the mini hdd. Is that the case or would a 500gb external drive work? I only use a tiny amount of my mini's hard drive so I don't feel like I really need 1tb. (Not even years down the line with my usage).