At the moment if a backup server is totally lost then you would use the "disaster recovery" feature to assign the affected websites to a new backup server. They would start making fresh backups from that point but obviously the historic backups would be lost.
If you have a tertiary copy of the btrfs data there is no official way to re-attach it but I think it could be done manually. You would create a new backup server within your cluster, attach the existing btrfs storage then, instead of using the disaster recovery feature (which assumes the historic data is lost) you would update the backup location manually in the Enhance database - we can assist with this if it ever happens.
