Yamaha DTXpress/DTXplorer/DTXtreme

I've recorded a few  midi songs to the DTXpress module 
and it works great. Its possible to mute the drum track 
to play along -but, althogh there are about 30 or so empty
song tracks, I can only record a song or 2 before I get
a "Memory Error" - This occurs because the all of the 
modules memory is used up. These songs are only about 30 - 50kb
Has anyone else had this problem ?  

Thanks 
Bill McGowan