Umm...he wants to transfer it to hdc, not hdb. That means going from primary master to secondary master. All that needs to be done hardware-wise is move the cable from the primary to the secondary IDE channel.
Software-wise, there are at least two things that need changing. First, you need to change your fstab. Any references to hda need to be changed to hdc. Second, you need some way to boot up hdc. This will depend on what you have on hda. If it's another Linux setup, and it has the grub bootloader, then you don't need to do anything. Grub will automatically detect the Linux partition on hdc and will offer it as a bootup choice. If hda has lilo, then lilo will need to be updated. If hda has Windows, you'll probably want to install grub onto it.