unplug the harddrive, and power it with external psu, then solder the psu wires right to the power switch so it goes on and off with the computer.. that is what i did with my classic. those vintage hard drives draw a hunk…

http://www.microcenter.com/single_product_results.phtml?product_id=0285941

these kits always come with a harddrive psu, for 11 bucks cant go wrong.