Best way to sync a local folder to a network folder regularly

What's the best way to sync a local folder to a network folder regularly (like once per day)? The local folder is actually a OneDrive Business folder (but since there is [ ] "Save space and download files as you use them", it behaves like a local folder). The network folder works behind a VPN, so backing up will fail if VPN is not running. Therefore it would be important that there was some reasonable error message if backing up fails so that VPN can then be started (manually, if automating that becomes too difficult).

In other words: What's a good backup software for backing up only changed files, when the target folder is an SMB network folder and files need to be backed up as they are (not converted to some compressed file)?

There are a lot of strategies that you can use.

You can use a simple command line or batch file using robocopy (a command built into Windows). There are also a number of free and paid for applications for the purpose of synchronizing folders and files either locally or over a network. FreeFileSync and GoodSync are just two examples of such programs.

Now that I have used FreeFileSync multiple times, I have to say it's a fantastic program. Thanks a lot! (Of course it doesn't automatically start VPN, but that would not be difficult to start automatically using other methods.)

Thanks for the follow up. Glad to hear that's working out for you.