Hi,
To me, it seems like a bug as it shouldnt still show the usb as if its still plugged in when its not?
All your music files have to be in /mnt/MPD/ to get recognized by MPD.
Rune audio mounts your USB storage device automatically in this folder under /mnt/MPD/USB.
The system is not designed to automatically remove this folder when you unplug the USB device.
The reason for that is that MPD rebuilds the library each time you add or remove files from that folder structure.
This is not a problem with small storage devices but is a pain with large ones.
We can try to make an optional setting (auto-unmount) like the automount one from the library-settings page.
Frank