I'm using the master/slave pins on the back of the hard drive in conjunction with a couple DPDT switches to "set" the OS before starting the computer. So each hard drive can be set Master or Slave from the front of the computer BEFORE turning the computer on.
Be sure not to flip the switches while the computer is running... could be a bad thing :D
Step 1: Mounting the Switches
Step 2: Building the Harness
I removed the "thick" wire from the audio cable because it was not shielded and moved the other wire over (finished product in the second photo). Very patient/delicate work, but worth it. By pressing in on the metal tab that holds the wire in place, the wire will slide out connector and all from the molded plastic. This allows you to change the wires position in the plug.
Using a utility knife I also shaved off the "clips" and alignment pins from the molded plastic connector. They won't fit over the jumper pins otherwise.
Step 3: Wiring the Switches
That is a little confusing to say but easy to see. The second photo explains. Red wires are the slave circuit and white is the master. As you can see the switches are wired backwards of each other. this allows both switches being thrown in one direction to indicate one OS.
In my situation both switches thrown to the right sets the hard drive containing Windows XP as master. Throwing the switches to the left sets the hard drive containing Ubuntu Linux as Master.
Step 4: Making the Hard Drive Connections
Step 5: Assemble
Advice for Windows/Linux users : Try not use the same model hard drive for both OS. Much easier to tell them apart if there a different brand or size. Reason being is that Windows does not like Linux... but Linux... well... it tolerates Windows. To explain, once your up and running in Linux you will be able to see, access and write to the windows drive. When in Windows the Linux hard drive is not accessible but is listed, should you attempt to open the Linux hard drive in Windows, Windows will prompt you to reformat the drive... obviously a bad thing. Easy way to avoid this "Potential Accident" is to disable the Linux hard drive in the Windows Device Manager so that it is not even listed when running Windows. May seem unnecessary, but it is my recommended "safety precaution". The instant I noticed this possible catastrophe I knew I had to find a "stupid proof" patch to keep myself from making that mistake. I almost did just that when transferring backups with external USB drives.