Holding down the power button to shut down, is quite brutal, and not advised on a regular basis, as it doesn't allow the computer to carry out it's usual shut down routine, or to save any settings that may have changed. Does the external hard drive have a power switch, David? If so, I would try powering the external drive off, before shutting the PC down. On mine, it won't actually start if the external hard drive is powered on, so they can obviously have some sort of effect on starting up and shutting down.