It took a long time to figure out what was really happening, including looking at a lot of code in samba.  The windows "bug", imho, is that it should not offer a login username/password dialog prompt when a server returns an "unauthorized" error as a result of trying to browse a Private share; I don't know why Windows does this and you can make it happen with a windows server so it's not strictly a samba compatibility issue.   Here's another confusing behavior: Let's say your windows logon na