My question is that how to add/use two hard drivers at a time in one computer, and how to transfer data from one to another after that. Waiting for answers.
you need to slave the second hard drive.. lok for the pins in the back of the hard disk on how to, read the info at the back
The safest and fastest way without messing the wires is to buy a cheap USB enclosure. All you have to do is to plug it to the computer's USB port.
I usually just plug the drive in then go to your OS's drive management tool and format the drive and then start using.