hdd not recognised


hello, i have a g5 imac 1.6. The 80gb hdd was failing so i had a spare and used super duper to clone the failing drive. It worked a treat but i needed more space after a while so i had a maxtor diamond 160gb which worked 100%. So i erased it same as before and used super duper to clone it. Problem is the g5 sees it only as an external hdd but when i install it into the mac nothing happens for about a minute, the screen stays black and the fans go berserk. Thanks for any help. Regards, bob