So I could have a play about with it, I deleted .mediabox, and relaunched the app with only one of my sd cards in, which has about 13GB (112 albums) of music on it.

the results os df -h .mediaserv are below. as you can see, now it only uses 56M, but still way too much. .thumbnails is empty by the way.

du -h .mediabox
0 .mediabox/playlists
0 .mediabox/upnpcache
0 .mediabox/youtube/search-cache
0 .mediabox/youtube
56.7M .mediabox/idtags
0 .mediabox/themes
56.8M .mediabox

hope this helps..

Thanks

Gaz