For the Medeli DD650 modules (Command/Crimson/II/DM10 MKII Studio/Forge) the options for recording are...
1. In Song menu, press the [RECORD] button, recording performance with accompaniment (on board songs) playing.
2. In Kit menu, press the [RECORD] button, only recording your drum part performance.
There are two methods to record: record as MP3 format or MIDI format.
1) With a USB flash drive, auto record your own performance as MP3 files, up to 99 user songs can be recorded to a USB flash drive.
2) Without a USB flash drive, auto record your own performance as MIDI files, up to 5 user songs can be recorded in the drum module
internal flash drive.
If a USB flash drive is plugged in, the recorded song will automatically be mp3 format and will be automatically saved to the USB flash drive...
These modules do not have the ability of recording to USB Flash Drive while playing a File from USB Flash Drive..
EDIT:
You can connect an audio source (smartphone mp3 player..any audio through a 3.5mm TRS cable) to the Aux input and in KIT mode..record.. and the module will output your performance and the Aux audio together as an mp3 and will save it to the USB Flash Drive..just remember to set all your levels in advance (no Aux input level on these modules) and set the metro/click to phones...