Hard disks to be made double

I have a spare hard disk. Not the external one but the one which will be fitted into desktop computer. I was wondering can I use it as a local drive for data storage. Is it possible? If yes how? How can OS control file system of second hard disk with no OS in it. will it be similar to USB ? I doubt that much simplicity yet. Nevertheless I have got a thought and would like to know if that is something which is achievable. It is okay to format spare hard dusk fully if it is required. Any suggestion?