
The file structure that worked on the Zumo 660 (and appers to work on the 396 / 595 is as follows:
When I use "---" this indicates a subfolder. A sub-sub folder will be ---/---
Garmin
---GPX
---userdata
JPEG
Music
userdata
No files are required, the music is filled in under the Music folder.
I know this still works because I am currently listening to music on the SD card, without any issue. (OK if I connect the phone then I do not hear music but that is another story!)