Possible suggestion
What I did in my install after adding a 2 tb storage drive, is mount /home/public/data to the new drive after copying my data to it; yes I know its not the linuxMCE way, my apologizes.
Also what I have found with previous testing (playing the fool), if you making changes to directories within myth, these changes are replaced by a setup script for myth (a perl script), on a router reload and on restart. You may need reflect you changes within this script, for me I did this through a simple if statement to reflect a local disk on an MD.