Page 4.
Page 4.
This is a rough diagram of the switch using a power cable splitter connected to the power supply.
Lay your power splitter cable out like this.

Cut your wires as shown below. You must keep your RED wire and your YELLOW wire in their proper position in the sockets. That's called continuity. They MUST keep their continuity throughout. If you don't keep continuity - ??.

On the power supply end, put a small piece of electrical tape over each of the four cut wires. We don't want the conductors to ever touch when we are finished.. After you place a small piece of tape over each of them, then wrap all four of them securely with one longer piece of electrical tape. Don't tape the other cut wires yet; they will have connections.

Below is a drawing of the underside of a DPDT (double pole double throw) switch. The red and yellow power supply wires each connects to a center tab. Drive 1 connects to a red and a yellow tab and Drive 2 connects to a red and a
yellow tab. Yellow doesn't show up good on the page so I'm using blue for yellow.
Measure how much wire you will need to reach from the power connector to where you will use your switch. This could be perhaps three to four or five feet. Cut your red and yellow wires, connect them to the power supply connector wires. Solder them and wrap each of them with electrical tape. Connect the other end of the wires to the center positions of the switch and solder them.
Before touching any electronic circuit boards, read the warning on Page 2.
Don't use a wire size that is smaller than the wire used on your Power Supply. I'm using 18AWG. My 'junk box' DPDT switch is rated 3 amps @ 125 volts.
You can add the switch to your existing wires without using the cable splitter. The cable splitter allows you to remove the switch at any time, leaving all your original wiring intact. The splitter makes it easy to change power supplies. If you sell your computer all you have to do is remove the switch. Since all wiring in the cable splitter circuit are identical to the power supply wiring, you have made no modifications to the original factory wiring.

The BLUE wire on the diagram is in place of the YELLOW wire.

YELLOW doesn't show up very well on the diagram but BLUE does.

The BLACK ink is used for clarity. All three wires on one side are RED and all three wires on the other side are YELLOW. I hope the black ink doesn't cause confusion. BLACK GROUND wires do not connect to the switch.
Warning: A friend gave me a handfull of these cable splitters - I've used three of them. The fourth one had a surprise. I was accustomed to making the switch so I didn't check the position of the wires on the splitter. I simply proceeded as usual. After building the switch and before testing it with the power on, I plugged the connector in and then checked my wire colors. For some unknown reason, the connector to the power supply had the red and yellow wires reversed. If I had simply applied power without double checking the wires I would have ruined a hard drive. Now, before I use a cable splitter, I plug it into its sockets first and check the wire colors, then I build the switch. Don't take anything for granted. 

The position of the Red and Yellow on these sockets may be reversed as you view them - it depends on which way the sockets are turned when you view them.

To be sure of your colors, look at the wires as they exist in your computer from the power supply to the hard drive.

It will be a help to draw a diagram. Know what you are doing.

Follow Red and Yellow from the power supply connector to the hard drive. Your Red and Yellow switch wires must be positioned exactly the same from the power supply to the hard drive. All you are doing is extending Red to Red and Yellow to Yellow. If you reverse these wires you will ruin your hard drive.