Rsync is a good way to create and maintain a "mirror" of specific folders on your unix system. It's not good for archiving, for cloning disks, or running a "full/incremental" backup system.
What rsync does is compare two folders, and syncrhonize them.
The following command will backup my home folder to an external disk called "/media/extdisk".
rsync -a /home/johnk/ /media/extdisk
Of course, life cannot be that simple. I have some huge folders with a lot of chaff that I don't need. First, I don't want to backup my Downloads directory. Nor do I want to backup my Freenet storage, which is 10 gigs.
I also have a 24 gigabyte Music folder, but don't want to scan that every singe time I run a backup. Conversely, I want the Desktop folder backed up frequently.
The typical unix way to handle this situation is to write a backup script. Here's my script. It's stored in the target backup directory, so it's not listed. I "cd" into the directory and run the script:
#! /bin/bash rsync -av /home/johnk/Pictures/ Pictures rsync -av /home/johnk/Sites/ Sites rsync -av /home/johnk/Desktop/ Desktop rsync -av /home/johnk/Documents/ Documents
The backup takes around two minutes to scan 24 gigabytes of data and back up the few new files that appear.
There are ways to execute this script when the disk is plugged in, but they differ based on OS. The system in Linux is called udev, and it's fairly complex, and I'm learning it.