I figured out why the files weren't updating - my mpd is running as mpd, not root ... In the last couple days I used recursive SCP to the root account to transfer folders of music into the USB drive. Transferring OGG files from Linux worked; rebuilding the library reflected the updates. Transferring mp3s from Windows didn't work; both files and folders are given root ownership, not necessarily with access for other users.
Here's a file that doesn't show up:
-rwxr-x--- 1 root root 12137099 Sep 23 22:45 '/mnt/MPD/USB/music/Various Artists/Classic Rock Broadcasts and Live Recordings - 100 Classics/68. Born to Be Wild (Live).mp3'
Here's a file that shows up:
-rw-r--r-- 1 root root 4836792 Sep 23 13:36 '/mnt/MPD/USB/music/David Bowie/Best of Bowie/Disc 1 - 1 - Space Oddity.ogg'
Then I changed the permissions a+rx for:
/mnt/MPD/USB/music/Various Artists
/mnt/MPD/USB/music/Various Artists/Classic Rock Broadcasts and Live Recordings - 100 Classics
And changed the permissions a+r for the mp3 file.
After that, the file was seen.