Gordon -- Squeezecenter allows you to specify a top-level music folder: Squeezecenter Settings | Music Folder. Both SC and the Squeezebox provide a Music Folder view that navigates to the directory specified in that setting. From there, you can navigate to subdirectories, or even different drives (using shortcuts).
For example, I have Squeezecenter Settings | Music Folder set to h:\audio\. When I select the Music Folder view in SC or on my Squeezebox, it takes me to h:\audio\. From there, I have access to a subdir LiveA-R. And I also have a shortcut, LiveR-Z, which redirects to another drive, j:\LiveR-Z.