Dont know how to reinstall hardrive. well i have a 500gb lacie external hard drive.and like one

Question

well i have a 500gb lacie external hard drive.and like one day my itunes said the disk was full or something and i didnt kno wat to do so then my dad said to eject the lacie from the mac,so i did but now i dont know how to install it back so now my computer is to full to do anything