Memory

I bought a refurbished 20gb hard drive along with a refurbished xbox360. This is my first time buying this system, but now my system tells me that the memory device capacity is only 13.9 and not the 20gb that I thought because the rest of the memory is being utilized for "system use."

Is this normal?
Will I have to format the memory device and delete everything in order for my device to show the 20GB capacity that I was expecting?
Or does every xbox360 take up 6.1gb from all hard drives for system use?

Thanks for the help.