Sorry I have the same issue but i read this topic and kinda get stuck ate the end..
redis-cli hdel usbmounts "/dev/sda1" -> returns 1
rm -r /mnt/MPD/USB/sda1-usb....your-folder-name -> what folder name, it's about a drive /stick, not a folder.. sorry but i don't get it. i ssh-ed into the rpi, so am at root@runeaudio.. maybe that is wrong as i only have these folders/files returned when i list the root.