The car's software may want a certain directory structure, .e.g. [groups]/[album]/[songs]. And it may only recognize certain file formats, .e.g. .wma, .mp3, .m4a etc. Usually the owners manual describes how the external storage should be setup.
I use SD cards and they are FAT32 with [Group]/[album]/[songs] directory structure and the song files are copied from my iPod directory on my PC - .m4a files.
When I first tried it, I had copied .wav files and the car player wouldn't recognize the songs.
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