[MacTUG] Catalina adds the ability to revert back after doing an upgrade

[Glenn Anderson]

>From https://www.macworld.com/article/3444585/8-hidden-features-of-macos-catalina.html

Revert after a bad upgrade

Have you ever installed a software update only to discover that it has broken other software you depend on to do your job? Catalina adds a new feature that takes a snapshot of the system disk just before applying an update, and allows you to restore your Mac to that state via macOS Recovery. (Restart your Mac and hold down Command-R to boot into macOS Recovery.)

This is a clever use of the snapshot feature of the APFS filesystem, but it takes up a lot of disk space (since your drive has to save very bit of your pre- and post-upgrade Mac). Snapshots are only retained for a day, and if you don’t have enough free space available they won’t be available at all.

Still, this is a really great feature—that ironically is being introduced in an OS version that intentionally breaks compatibility with more than a decade of macOS apps. At least you’ll be able to roll back after the next one.