Nicklock makes one, called a "Hard Disk Selector". It's a physical switch. But it will completely disable the drive not being used, so sharing data between the two is not an option.
http://www6.tomshardware.com/storage...727/index.html
What brand of hard drive are you using? Certain drives, especially Quantum, may not work so well as I've had ones that would only work as a master when on a certain ribbon location.