Missing space on hard drive after install

Hi,

recently wiped my hard drive to install ubuntu, but 20gb is missing from the drive. The hard drive should be 320gb, but is currently less than 300gb. I have tried wiping the drive again by installing lubuntu, but the problem is persisting. Had a look at the forums, I'm just booting through a live lubuntu CD to see there is anything else I can try.

The computer works fine apart from the hard drive issue. I'm not sure what information I need to provide, but gparted is only showing less than 300gb.

Thanks