There are a lot of reasons why this error occurs, but the message is actually deceiving. Rarely is the device name actually already in use. The typical reason for receiving this message is that your machine simply cannot connect to the other machine for whatever reason.

The most common reason I have discovered is that a port is being blocked via a firewall. If you have enabled the MS built in firewall, then you will definitely get this message.

Windows uses UDP ports 137 and 138 and TCP port 139 for network communication. Unfortunately these are the worst ports, from a security standpoint, to have open. You can usually get drive mapping to work by opening the MS Firewall on UDP port 137.

To get to the MS firewall in XP, right-click on your network neighborhood icon and select properties. Right-click on the internet connection that you are using and click on the “advanced” tab. There is a checkbox for the firewall. If it is enabled, click on the settings button. You open up the ports by clicking on the “Add” button and then specifying, for example:

Name for the service: (whatever you want)
Address of machine: localhost
Select UDP
Internal Port: 137
External Port: 137