A backup you have never tested is a hope, not a backup. The aim is to be able to answer one question confidently: if this computer stopped working right now, what would I lose?
- Keep more than one copy, and keep at least one of them somewhere other than your home or office.
- Use the built-in tools where you can — File History on Windows, Time Machine on a Mac.
- Add a cloud backup for the things you genuinely cannot replace.
- Check occasionally that backups are still running. They fail silently more often than you would like.
- Actually restore a file now and then, to confirm the restore works.
Note that file syncing is not the same as backup. If a file is deleted or encrypted by ransomware, sync will faithfully copy that change everywhere.
This is general guidance, not a guaranteed fix. The same symptom can have different causes on different machines. If you are unsure about a step, or it does not help, raise a ticket and tell us what you tried.