Back up before you upgrade (every time)
The rule is boring and I still break it. So the fix is mechanical: the upgrade command lives in a script, and the first line of the script makes the backup. If the backup fails, the upgrade does not run.
Keep the backup somewhere the upgrade can't touch. A copy on the same disk you are about to reflash is a note to yourself, not a backup.
Also write down the version you are leaving. Rolling back is much calmer when you know exactly where "back" is.