|When using pure DOS you have at least two problems which are insurmountable:|
DOS cannot access NTFS partitions.
DOS cannot access plug-in USB devices except for basic vital hardware like keyboards and mice.
You should be able to access the files on your MP3 device by using Windows, provided you have installed the necessary driver from it's manufacturer if they have one.
Alternatively, Windows Media Player may be able to access it by clicking the "Sync" tab.