Using Windows on another HDD on a Mac
You should be able to simply boot up off of the Windows USB installer, and install onto the second drive. You may need to temporarily remove the OS X drive before installing, as I have heard windows doesn't like seeing other OS's. You may want to do anyway, just to make absolutely sure you are not harming the OS X install.

Once you have it installed, you will want to install the Bootcamp drivers. You should be able to download them online.