Doh! I was focussing on the return value of zero -- i.e. nothing was written. In most cases, that'd be an error. Of course, since it attempted to write zero bytes, it's not (well, not in write, anyway).

Is there any music on this disk at all? If it's attempting to write the database (or playlists) file, then it could be caused by having a missing or corrupt root playlist (100) file. Unless, of course, it's attempting to write the tags file, in which case, something more serious is wrong, since the first half-dozen (or so) lines of that file are canned and are written from a const string in the player binary.

The fact that the error message talks about the tags file can be a red herring, since that's the file the player uses to detect a missing database.