This is not a backup, just a copy…
If your server suffers an electrical shock, you’re done 
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The third disk is a external USB drive. I hope to buy a NAS server in the future.
Hope you’ll do asap
Just to put in my thoughts on this: If you mount an external volume locally, you do have a valid backup.
I happen to have a 1TB webdav (owncloud based) offsite diskspace. I can mount that using davfs to a local mountpoint.
I would love to see any of the backup tools make it possible to backup to that local mountpoint. Ideally, mounting occurs prior to the actual backup and umount when the backup is finished.
I saw a howto by @Nas, but I would rather not tinker with a feature that is meant for USB devices (I might want to do a backup to USB at any given point…)
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