How to keep two different volumes on a network sinced

[ReneZ]

Hi, maybe a strange question, but I operate a server from my QNAP NAS and from a RaspberryPI set-up.

I would like to keep the music directory on the QNAP and on the USB harddrive attached to the RBPI sinced (basically make sure that what I add to the one gets copied to the other). There is no 'master drive' and meantime both drives have different bits of music on it, so simply copying one onto the other would not work.

Does anyone has ideas or experience dealing with this?

Thanks, Rene

PS If I have to do two steps (first ensure both drives have the same content, then create a set-up to have updates applied to both volumes, that's OK.