Does mocp run well in console/terminal when you start it manually?
Is sdb1 a usb device, and if so is it being detected and mounted in time to be used?
Is your system still set up to startx X automatically? There may be a conflict in that case, since mocp tries to create a curses interface by default. Try mocp -S -m /mnt/sdb1/music, to start it without the gui.