Dirvish is an easy to use backup system based on rsync. Having recently setup an external encrypted usb drive to store backups, mounted at /mnt/backup1, I setup backups for target directories on my local system using Dirvish (of course, if it were not in early development, I would use Snap but alas its too early to trust critical system operations to it). The entire process is fairly easy, there being two files which you need to modify, as well as creating a cron job to run dirvish daily. I'll keep it brief and simple here, use this great guide if you want more info. Before we begin, recognize a 'bank' is a directory where multiple vaults are stored. A 'vault' holds the configuration and the actual snapshots of the filesystem(s).
A few months back I wrote an article on how to create and access an encrypted hard drive in Linux. Recently I've been setting up a large external usb drive to store nightly backups and couldn't figure out how to load the unencrypted device /dev/mapper/enc from the encrypted device /dev/sda1 post-boot (eg the usb drive wasn't turned on until the computer was completely started up). Lo and behold the answer was right in front of me, in my very own article, eg. step 3 or running the cryptsetup command:
cryptsetup -c aes-cbc-plain -d /etc/enc-key create enc /dev/sda1
I was under the impression that this altered the drive somehow, but does not in any way, it merely loads the unencrypted partition, from which point the user can mount the filesystem. I suppose a warning should go here, because I presume its possible (tho I have not tried it) to load the encrypted partition with a different key and thus the unencrypted partition will be "illegible", eg. the computer thinking its just random data. Any reads or writes to it will most certainly destroy any stored, encrypted information.
Anywho, it took me a little while to figure this one out, so I figure it might be useful for anyone else in the same situation.