The reason it booted from the disc after you zeroed out the hard drive is because there was no boot record on the hard drive. Since it couldn't boot from the hard drive, it checked the next device on the list, the optical drive. Finding a boot record there, it booted. You could have still booted from the optical drive by putting it first in the boot order in CMOS or by selecting it from a boot menu accessed by pressing F8, F10, F11 or F12 (depending on your computer or motherboard manufacturer) at the POST screen and selecting your optical drive as the boot device. There is NOTHING on the hard drive that would have prevented you from booting from the optical drive.