Mac OS X sometimes stores extra metadata inside a file's resource fork. Since the file forking scheme is often different or missing on other filesystems, OSX will create a hidden companion pseudo fork file instead. These files have the same name, but star with "._" (period underscore). Picard should ignore these files when scanning as they are often zero bytes in length, or if they have any data, it would be incomprehensible to Picard anyways.

I scanned a folder on an ExFAT thumb drive with MP3's copied from an HFS+ volume. Picard came up with an unreadable "._song.mp3" for every "song.mp3" in the list.

It may just be best to have Picard ignore all hidden files (however the local operating system defines hidden file).