install an external hard drive on volumio

hello (sorry for my English, I use google translate).
I use rpi3 and volumio with satisfaction. I can install music from a nas synology. in fact, I would like the music to come from an external usb drive.
when I connect the external drive to the rpi and I turn on volumio, volumio scans the external drive and the music is installed. when I turn on volumio and rpi after turning them off, volumio scans the disc again.
is there any way to install the music of the external disk as if it were a nas. during a volumio test, a long time ago, the external disk appeared in the list of music supports next to the nas. since then nothing.
What is the solution? Thanks for your help. dom2