Adding games to HDD via Winhiip - not seeing full drive

I have added a 320 GB hard drive to my ps2 but when i go to use winhiip to add games to the drive it only sees it as 120 GB - I have tried reformatting the drive several times but still come up as the same result.

Any ideas why ?

Is there a better program to use on windows to add games to the hdd ?