It could be your MicroSD card has gone corrupt.
Try putting a fresh _ds_menu.dat and .nds game and see if it works.
If it can't transfer then its MicroSD card.
If it does transfer on and the R4 can't read/find the files then its your R4.
Sorry 'bout that.
I'm sure R4 DS was released first but the M3 DS Simply was hyped and we had heard about it for a few weeks.
There was news about the M3 Simply but the R4 DS was just launched straight into the market just before the M3 DS Simply.