When mounting external files the database update did never finish for me.
Only a small part of the mounted files was imported.
This took me quite some time to fix and since i could not find the solution through my search strategy, i thought to share this here. I hope someone finds this useful.
Looking at the mpd.log i discovered it always being m4a files, that had the mpd scan choking. Finally i found the files disrupting the import had images (artwork) included.
I used mp4art of the mp4v2-utils package to remove the artwork.
This can be done on all m4a files like so:
find . -type f -name \*.m4a -exec mp4art --remove -z {} \;
Hope this helps.
Cheers