There was an article on this site describing how to add an SD card to the libraries so contents are visible in Music and other apps on Windows RT.
I tried it and it worked really well on my Asus Vivo Tab RT. I say "worked" because my tab developed a problem with the screen and had to be repaired. In the process they blanked my hard drive and I had to set everything up again.
When I tried to do this a second time I got an error of 'invalid parameter' when I tried to change the drive path.
I'm really hoping you have a solution to this as it so drastically affects how useful my tablet is.