Seeing music on OS X from Ubuntu

After my recovery of my Mac HDD, I'm now going through the processes needed to get it back to where it was (or better).
However, I always seem to have problems being ble to play music from the OS X partition in Ubuntu. The "rules" as I understand are simply that the /Users/paul/Music folder must have, at least, READ privileges for User, Group and Others (and needs to be the same for all underlying files). If you need to add music to this folder from Ubuntu, then Read&Write privileges are needed.
Well, I did this but still, music would not even load in either Rhythmbox or Amarok in Ubuntu.
Then I found, in Ubuntu, that /media/osx/Users/paul/Music/My Music had owner 501 and dialout as group-owner. So, I changed this to paul:paul (including underlying files) but still no good.
Only after switching between OS X and Ubuntu a few times did the media players finally recognise the music collection and I could play it (without using root).
Very strange, like it had a mind of its own.

I noticed during all of this messing around that unknown users and unknown groups were popping up now and again in the /Users/paul/Music Sharing&Permissions box. Have no idea where these come from but maybe I should post to the forum about it.