My idea of a backup is as follows:
- I have an external hard drive (same size or bigger than the drive in my live-system)
- Once a week or once a month, I plug that drive in to my computer and run a backup-command or click on a button in my backup-program
- The backup is incremental, meaning the very first time it runs it will take a long time and every subsequent time it will only copy the data which changed
- Once the backup is complete, the external drive goes back into its drawer/shelf
- If the drive in my live-system dies, I replace it as soon as possible and will be able to retrieve my data from the external drive.
- I don't really care whether the external drive is bootable or not. If it is, that's nice, but it doesn't have to.
Back when I had a Mac, I used this method to run backups. It worked like a charm and if I still had a Mac, I'd probably still do it that way.