Tonight I tried to put music on my RCA M6204-B MP3 player but when I tried to play it, it said the music section was empty.
TL;DR If your RCA MP3 player says the music section is empty even though you know it's not, make sure the MUSIC folder has at least 14 audio files. If that fails, split your music into at least 14 subfolders.
This issue didn't make sense but I thought maybe I needed to use RCA easyRip. After installing that I had the same problem. I tried putting music directly in the MUSIC folder and a subfolder in MUSIC, making sure it was formatted as FAT32, formatting it in Windows and from its Settings menu, and trying MP3's from various sources. Nothing worked. I found other people online with this issue but most of the results were on fixya.com where they suggested calling a support number.
Fortunately, before clearing my player I had saved everything to a 7-zip file. I deleted everything and extracted the file onto the player and it worked. I thought there was some kind of database that the official RCA app populates when it transfers music, but the manual says you can copy music in Windows Explorer so that didn't make sense, but it was still worth trying. RCA easyRip was able to copy the music and list it when I restarted the program, so it should have worked but it still didn't.
Finally I copied everything from the 7-zip file onto the player again and decided to delete some of the music folders until the player said it was empty. I eventually discovered that the player requires the MUSIC folder to have at least 14 files, or folders with files, before it will recognize any music.