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

[Garnett]

Im not saying it reserved 30 gigs of space i just used that as a number,its 7 as mentioned.

 

I know it needs to reserve some space,but i had 30 gigs left,30 gigs thats WITH reserved space,why cant i use those 30 gigs? When i try to install a game or something it wont work.