Help! Raspberry Pi/LMS confusion

I'm new to the whole command-line interface thing. LMS is running on my new Pi, but I can't for the life of me figure out how to point LMS to my library which resides on a USB volume connected to the Pi.

I can "ls /media/USB_PI/iTunes" and see all the folders and music files in the Terminal(Mac) and in the Finder if the drive is connected to the Mac, but LMS doesn't see anything-- and resets itself to "/mnt/music" every time I try to change it.

A puzzlement.

Help?

Thanks in advance.

DK