I believe where turning off one thing results in a connected thing going to standby or off, it has to be because the connected thing is monitoring and knows to turn off when there seems nothing connected to its data cable. I'm making a reasonable guess here. My suspicion is that if it is supposed to happen (and the fact that it has been doing so suggests it should) and now stopped it is most likely to be the hard disk monitoring that would have failed. Although I guess it's not impossible that the TV started still looking connected when it no longer is. Take a web search for the hard disk you have, see if the spec/functionality includes auto standby.