Had the same problem with two out of three of these units my family bought, after about a year of use, think the problem is the power supply, the little box between the mains and the unit itself, apparently its producing enough power to run the unit and show the camera's but not enough to run the HD, I too bought a new HD for mine thinking it was this that caused the problem, try a new lead or you may have one connected to a games console or something to test to see if this is the fault, Good Luck.
