Remote Copy

I would recommend looking for NAS devices that support rsync, which is a bandwidth-efficient way of mirroring files. I use it for local backups from a DNS-323 that has been rooted. This works fine for me but the device is quite old now and I would expect to find a modern NAS that provides rsync in a more user-friendly package.

You will find things easier to setup if you can have one NAS be the master. Also, you will have to setup dynamic DNS and your router firewall so that one NAS can find and connect to the other one.

I would say this is straightforward but not exactly simple. An alternative approach would be to use some form of cloud storage that both devices use as a master reference. Quite a few NAS boxes have some sort of cloud sync, so this might work out-of-the-box but I found cloud storage charges for even moderate music collections to be too expensive when I last looked.

Netgear has an application that does just this with some of its ReadyNAS units.  Have a look on their website - depending on what you call 'cheap', they are not cheap !!

I use two ReadyNAS units, but the second is only for disaster backup and is in my garage.  My garage is in the same country (garden) as my house though, so it's all done over CAT5.  But the Netgear app is designed to run over a remote link/network, so it's all quite feasible.

Thank you. It gives me a few places to start reading.

My next challenge is to get a phone line installed in France. A logistical nightmare no doubt.