Share my lan server external drive through the router

I have a laptop and desktop connected to a di-524up router, then into an adsl modem in bridged mode. The external drive connects via ethernet to the router, usb to the desktop. I would like to share the drive between the two machines, but not the outside world. Am I better to use samba or the logon/password each folder method. Each piece of hardware is connected to the router via machine numbers if that helps in any way.