If you turn off your hard drive in battery saving mode or just say you leave your laptop on overnight but want a program to start downloading at 5am. How does the hard drive switch itself on again and what prompts it to?

Windows will automatically spin-up the hard drive whenever it receives a read or write request for the hard drive. Then, after some period of inactivity (whatever interval you've set) it will stop (go to sleep) again until it receives another read or write request. The thing to remember is that although the hard drive is stopped to save energy and wear and tear, the memory is still active so your system is "awake" and executing instructions. -- kptech

Settings in Battery OP mode will have no effect when the Laptop is plugged into the AC adapter. If this is for M$ Windows Auto update you can configure this through Auto update settings. You will have to enable an appropriate "WAKE ON" setting depending on your Internet Connection. Something on the order of "Wake on Lan" Wake on Modem" There is nothing to learn from someone who already agrees with you.

[quote] Windows will automatically spin-up the hard drive whenever it receives a read or write request for the hard drive. Then, after some period of inactivity (whatever interval you've set) it will stop (go to sleep) again until it receives another read or write request. The thing to remember is that although the hard drive is stopped to save energy and wear and tear, the memory is still active so your system is "awake" and executing instructions.[/quote] So does this mean the laptop runs a bit cooler or does it not make any difference? How much energy would it save?

If you were to translate the savings into $$, I don't think you'd save more than a few cents a month. The real savings is in runtime while operating on the battery. By powering down the hard drive, monitor, etc., while they're not in use, your runtime on a single battery charge can be extended by as much as several hours as compared to leaving them run all the time. Most systems these days can also "step-down" the processor speed while on battery power. This will reduce the amount of heat produced, but again, the real benefit is that you'll be able to run longer on a single battery charge. Most if not all of the available power settings can be accessed via "Power Options" in the Windows Control Panel. The manufacturer of your system may have also included their own utility to change these and other more advanced settings. -- kptech