Please exit to shell, and let me know what the following does. I suspect that you may have partitioned and formatted the music file system on the 100GB, but not added the directory structure...

q (get to shell)
ls /drive0
ls /drive1

Meanwhile, what is your prefered end configuration? Do you want to have this player with a 20GB master and 100GB slave? Do you want to transfer everything from the 20 to the 100? Do you want to have the 100 alone as a boot drive? This will determine how / if you need to copy audio and configuration files, and if you need to move drives around and apply a .upgrade file...