Your player does not read the internal memory & external card separately. It instead reads the ID3 tags embedded within the audio files to glean the track information. It then displays this info (Artist, Album, Track, etc.) in lists on the player. Music stored in both memory locations will be combined into the same listing alphabetically so it doesn’t matter where it is, you’ll be able to find and play it.
If you cannot find files you have stored on the card, look to see if you have an ‘Unknown’ category in your Music (Artist, etc.) lists. If so, and the ‘missing’ music is listed there, you have some tag creating or editing to do so the player can read them.