That's what I thought, but that's not what it says in the command reference
http://www.kixtart.org/index.asp?p=commandRef#_Toc146265755

"On Windows NT and on Windows 95 OSR2 and later versions, Drive does not have to specify the root directory on a disk. On these platforms, the function accepts any directory on a disk."

And it doesn't work on Windows XP Professional and Windows 2003 Server.
So... Is it a bug or something ? :-)