In short, NO. You need to use a piece of freeware to dual-boot, or, if it's supported, you can choose a dual-boot option in Linux, during the install. JUST INSTALLING LINUX ON A SEPARATE HARD DRIVE AND CONNECTING IT TO YOUR COMPUTER WILL NOT WORK. When the computer starts up, it refers to a file called "boot.ini", unless you're running Vista or 7, and from there, it knows what OS to start. If you just plug in a hard drive with Linux on it, the boot.ini file is not changed, and so it will just start Windows like it would without the other hard drive.
Enjoy and Please Rate!
Sincerely,
pwn_n00bs