Hello, my problem is that. Hello, my problem is that i can't seem to get my second (built-in)

Question

Hello, my problem is that i can't seem to get my second (built-in) 140gb hdd to store any data?! My first drive is almost completely full and keeps coming up with the low disk space warning! I was just wondering do i need to do something to 'activate' the drive or will it just automatically start using it?! Many thanks, Jonny