Can't access eternal network drive

I have 2 computers both running windows xp sp3. Both computers have all of the same services running and startup. Both computers are on the same network and CAN access the public documents folder from the server. However, only 1 computer can access the external network drive. The other gets an error of no permission to access it. Why can 1 access the external network drive and not the other? They both have the same configuration except being the same brand of computer. The network has 2 other clients running windows 7 which work fine. The server is running windows 7. None of the computers are running a firewall or other security. Any ideas?