I think even if you are installing onto the first 8GB, thats not enough - as Mike said, the drive should be partitioned, with the first partition being no bigger than say, 7.7GB. I tried installing OS X onto an empty 120GB HDD in my tray load iMac once, and it wouldn't allow me to install, as even though the drive was blank, and therefore the first 8GB was free, the drive hadn't been partitioned as such.