I keep a “master” music library on a NAS server and my Volumio music library on a USB stick plugged into my Raspberry Pi 4 based Volumio named “volumiotgg”
On my Win 10 pro PC, I can access the Volumio’s USB drive by typing “\\volumiotgg\USB\MusicLib” into the Windows file explorer address bar and dragging and dropping files from the server at will.
But, I find it a lot quicker to use a program called “Beyond Compare” from a company called Scooter Software. I presents two windows to compare to each other and so I put the Master Music Library in the left window and the Volumio USB music library in the right pane and let the program show me the differences between the two.
Then I just tell it to sync the two folders right to left and my Volumio is updated to match my master library. I t has been very convenient to do it this way.