I have 3 laptops a DV6, a DV5 and a DV7 with the same problem. Without a HDD cable they will post. With a HDD cable they will post. As soon as a drive is connected they will not turn on. Disconnect the drive and wait a few seconds and they will post. Tried 3 different drives and 2 different cables.
If you have the solution I would like to hear it. If you have a theory that starts with maybe I would not. Thanks.