Time Machine settings excluded many wanted files

Jim Mac37 wrote:

I did not set anything in Time Machine under 'Exclude from Backup' but somehow it was done.

A seemingly random selection of odd fonts, some files I could not identify and all user folders were not backed up. Then the machine failed catastrophically and as it had an SSD no recovery was possible.

Why and how do I avoid this happening again?

I have replaced the desktop unit with a MacBook Pro BTW.

To prevent this from happening again:

(1) Test your Time Machine backups. Try restoring some random files and folders, or maybe some of the ones that matter most to you. You can also inspect the Time Machine backup sets directly in the Finder and see what has been backed up.

(2) Have multiple backups, and also utilize at least two different types of backups. You can have multiple Time Machine backup drives, I had three on my work computer, two in the office and one at home. They take turns backing up when you have multiple such backup drives. But importantly, also utilize different types of backups. So in addition to Time Machine, use a "cloning" type backup (examples include CCC and SuperDuper, but there are others) and an online system is a good complement as it is stored off site. You need not subscribe to an online backup service, but using something like Google or Dropbox or One Drive cloud storage for your most important files (be sure to select the option to keep a physical copy on your computer(s)) is good insurance to avoid vulnerability to fire, theft, water damage, pets, physical damage to the computer (dropping it or dropping something on it).

I have never encountered a situation where Time Machine didn't back up key folders like user folders. Perhaps the machine was starting to fail gradually, before it failed catastrophically (I'm speculating).

Jim Mac37 wrote:

Yep, should have inspected the Time Machine backups and might have found the issues.

Maybe, but having multiple backups and types of backups protects you.

Now have an online backup service.

That's good, but you should have at least two backup types. I have three: Time Machine (which has NEVER failed me or had any anomalies, and I've used it many times to migrate to new machines or recover selected files); SuperDuper "clone" backup; and cloud storage. Online backup services are good but they can be impacted by setup mistakes or other issues, nothing is 100% reliable.

But how to stop it sneaking in?

Not sure what you mean here. Your other computer failed so maybe it's not surprising that other things did not work as planned. But if you have multiple backups and types of backups, you can tolerate on or even two not working as planned and you are still protected.