My computer had almost no memory, so I bought an external. Is there a way I can copy a game (Guild Wars, if it matters) onto my external harddrive and still be able to access it as if it was on my internal harddrive?
hmm no you can't unless you want to make a separate partition onto the harddrive but seeing that you have no memory i recommend you move all your media to the other drive and form there your computer can still access it or you can consider upgrading your harddrive/computer.
