Whats the point of a bigger HDD if you cant use it?

[Garnett]

Sabby e17 i understand that but out of 120 they automatically tell you what you cant use.

 

Say if i had a 20 gig it would say 17 gigs free,so you can use up that 17 but my 120 said i had 30 left and i couldnt do anything.