Batch command to log on to a mac

I have a batch file that xcopies data across my home LAN from one hard drive to another for back up purposes.  It works fine between the PCs, but the addition of a Mac recently is causing me a problem.  It is probaly not because its a Mac but Apple insist on shared drives being password protected.  So from time to time the Mac insists on a username and password.  If I log in from Windows giving the password and then run the batch file it works.  If I just run the batch file sometimes it fails because it cannot cope with the login need.  Can I add a line to the batch file that connects me to the Mac either every time or when needed?  I have scoured through various permutaions of 'net use' and 'net logon' to no avail.  Any suggestions gratefully received.