I want to get a hard drive which then I will use it to install Ubuntu 12.10 using the Windows installer. Is it possible to install Ubuntu on an external drive then plug it out for later use and back in so that I can use Ubuntu again? I don't want to mess up my already installed Windows 8 software.
In theory, yes. You will need to install the bootloader and grub on the Windows drive though to avoid issues with dual booting. Then even if you unplug the drive, you should still have a functioning grub instance before you plug the drive in again. Just be aware that this is a bad way to set up dual operating systems. ie. you might as well do it on the same drive.