Since the hard drive is inside your PC, it is not portable. If you wanted to transfer data from your hard drive to another PC, you would have to copy your data to a floppy disk first, and then take that floppy disk to the other PC.
newer removable hard drives> are available, however, that allow you to move your hard drive from PC to PC. You can even make your permanent hard drive removalbe with a special kit, if you desire to do so. More on this a little later.