Can I replace my slave hard disk with another while the PC is on?

No

Nope, not if it's IDE. I've heard something about being able to do it with SATA, but I wouldn't recommend it.

Now, if you mean like a BACKUP hard drive, you can do it if it's USB or Firewire.

If you want to be able to swap drives live other than your boot drive... install them into external USB cases. There are many on the market reasonably priced. Make sure the case supports the same technology as the hard drive.

It is possible, but I would also not recommend it at all. This could damage your computer and potentially cause future or immediate damage.