I have a NAS drive with two internal HDs set up in RAID 0 (seen as one complete drive) and two separate external HDs as backups. I plan to keep the backups off site.

This means intermittently I want to copy the new data from the NAS to a third portable HD to the site of my backups and copy the new data over.

Without simply lumping all new stuff into a separate folder new and then sorting it manually twice (for the NAS and backups) is there a way I can create an image(?) of the backups to compare the NAS contents against and thus only copy the missing (new) files to the third portable HD using the 'sorted' file structure?

On-line remote backup is not an option.

2 Answers 2


Your best option in terms of performance would be to use rsync from the command shell on your NAS to sync NAS HDD to one of your external drives. In other words, you would plug your internal drive, then rsync NAS HDD onto that drive and take latter offsite.

Alternatively, you can do the same from any machine that has an access to NAS, but when a file will be copied from NAS to ext. HDD, it will go through this machine. Not ideal in performance terms, but might be easier to set up if you aren't comfortable with NAS shell. For this option, the keywords you are looking for is folder backup or folder sync. Assuming you are on Windows, you can look at FreeFileSync (make sure to use older versions that aren't bundled with adware), Bvckup 2 or SyncBack SE. You can also use Windows own robocopy with /MIR flag if you want zero dependency on anything outside of bare OS.

  • I am not asking about performance?
    – myol
    Nov 10, 2015 at 14:11

I have a similar situation and I am using now Beyond Compare. Thare is a feature called "Snapshot". So you can take a snapshot of your external backup drive. After that the drive does not need to be connected to your computer. --> You have the snapshot of it, which is a representation of your data. (Of course without the contents)

Then you have the option in Beyond Compare to perform a comparison against this snapshot, compare and show only the differences and also to copy only the different file to a separate location.

Here you find more information about that feature: https://www.scootersoftware.com/support.php?zz=kb_sneakernet

The best thing is you can automate all of this process with scripts so that you do not have to do that all mannually.

I am user Beyond Compare for Windows 10 but within your licence you also can use is for Linux.

I hope I could help a bit.

Greetings, Alban

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