In 2005, Sony released its second-generation Hi-MD devices offering native support for the popular MP3 format (earlier, SonicStage would transcode MP3 files to ATRAC format before recording on the disc).
Sony's MP3 file support still means that the MP3s themselves had to go through SonicStage to be put on the device, and could not just be copied on the discs outside of SonicStage as you can with data files on Hi-MD (or Hi-MD-formatted MiniDisc media).