Hi Marc,
I think you can solve this problem by applying the following steps if you are using anonymous access. If you are using Windows Authentication see
Microsoft's KBA Q197964 instead.
1. Open IIS manager of the server (call it MachineA) that the virtual folder is created on (ie. the web server for your external visitors). Open the properties of the virtual folder and edit "Anonymous Access and Authentication Control" settings on the "Directory Security" tab. Make sure "Anonymous access" is checked, then uncheck "Allow IIS to control password" and enter a new password for the user IUSR_MachineA.
2. On the server (call it MachineB) that your resources (main site) reside, go into Local Users and Groups (under Computer Management) and add a user named IUSR_MachineA with the same password you entered in the first step.
3. In Windows Explorer on MachineB, right-click the share (ie. to which the virtual folder points), hit Properties, and on the "Security" tab, add the new local user IUSR_MachineA, give "Modify" permission and apply.