This question is an exact duplicate of:
My Lubuntu v12.04 (I'm not using the newest one because my Laptop doesn't support PAE) doesn't boot when the USB I used to install it isn't plugged in. I've overwritten my Windows XP with Lubuntu and the hard drive is the top priority in the boot options. I can even remove the USB while Lubuntu is running, it just needs it for booting, which is really annoying. What am I doing wrong?