This is referenced in various virtual directories within IIS on our Exchange 2003 server. Googling leads me to believe it's a holdover from Exchange 2000, but that path doesn't function in any way that I can find.
In case it helps, what I'm trying to do is enable replication for public folders to the Exchange 2007 server. Troubleshooting steps for the error I'm getting indicate a problem with the IIS settings for the Exadmin virtual directory. This odd path seems like a good thread to follow.
(tagging is sparse at the moment because I'm not sure to what I'm going to attribute this scrap of history)
Edits... This isn't a SBS server, and we don't have any (though there may have been before my time). It's a 2003 Server with Exchange 2003. There's no M:\ drive, nor anything mapped on that server, though I see this mentioned frequently. Public Folders appear to be functioning, as well as other IIS components (such as ActiveSync) which reference this odd creature.