Kopia uses content addressable storage. So basically when it copies things, it only copies what data is new. Files that haven't changed will not be overwritten.

You kind of need to run the verification command on both the source and the "backup copy" for maximum paranoia. If you're running it on a local copy, that should be a relatively fast process as you don't need to download stuff.

You'd basically connect on the command line to the copy you just updated via sync-to and then ask kopia to verify 100% of the file integrity ... it should then run through everything and make sure it matches what's supposed to be there. I'm not sure how you fix it if it detects something wrong, I've yet to run into that ... I'm sure there's a way 🙂

You could also use two backup drives and sync to both, then if you get an error restoring a particular file from one, you could in theory restore it from the other. A ZFS cluster with redundant copies and/or a RAID-1, RAID-5 or RAID-6 style setup could also help ... but most people aren't going to run an entire NAS just to turn it on periodically and backup their data "offline". Most people are going to be better served (IMO) by using cloud storage like B2 (where bitflips aren't really a concern) or a NAS (where bitflips similarly are a minimal concern, ideally in another location) with a periodically updated offline copy (on say an external hard drive) should be enough to protect most people's data well.

Also going to like to what I'm talking about:

• https://kopia.io/docs/advanced/consistency/
• https://kopia.io/docs/advanced/synchronization/