What a Backup Is
Scope: the definition everything else builds on. Three things get called "backup" that aren't, and the difference only shows up on the worst day.
The one-sentence test
A backup is a copy of your data that survives the loss of the original — including the losses you cause yourself.
That last clause is where most setups fail. Walk through the usual suspects:
| Thing | Protects against | Does NOT protect against |
|---|---|---|
| A second copy on the same disk | Nothing much | Disk failure, theft, ransomware |
| RAID / mirrored drives | One drive dying | Deletion, corruption, ransomware, fire — the mirror faithfully mirrors your mistake in real time |
| Cloud sync (Dropbox, iCloud, OneDrive) | A dead or lost device | Accidental deletion and ransomware, which sync to the cloud within seconds |
| A real backup | All of the above | — |
Sync deserves a closer look, because it feels like backup. Sync's job is to make every device the same. Delete a file on your laptop and sync does its job perfectly: the file vanishes everywhere. Some sync services keep a few weeks of deleted-file history, which helps — but that's a grace period, not a backup strategy.
What a real backup has
Four properties separate a backup from a copy:
- Separate. Stored somewhere the original's disasters can't reach — a different device, ideally a different building (next chapter).
- Versioned. Not just today's files, but last month's too. The document you mangled on Tuesday and only noticed on Friday is recoverable because the backup kept the older version.
- Automatic. A backup that depends on you remembering is a backup you'll find was last run in March. Humans are the least reliable component; schedule it.
- Tested. An unrestored backup is a hope, not a backup. The checking chapter exists because drives fail silently and formats rot — you find out your backup works by restoring from it before the emergency.
The failures you're actually defending against
Ranked by how often they really happen, not by drama:
- You. Accidental deletes, overwriting the good version, a cleanup command in the wrong folder. This is the most common data loss by a wide margin, and the one RAID and sync are worst at.
- Hardware death. Drives and SSDs fail without negotiation, sometimes without warning.
- Malware and ransomware. Encrypts what it can reach — including any backup drive that's always plugged in and writable. (A reason backups should be append-only or regularly disconnected; both tools in this guide make overwriting old backups hard by design.)
- Theft, fire, flood. The whole machine, gone. This is what the off-site copy is for.
What this guide's setup gives you
By the end of the restic chapters you'll have: encrypted, versioned snapshots on a separate drive or server, taken nightly without you, pruned so the disk doesn't fill, and checked regularly so silent damage gets caught. That's all four properties. Everything after that — Borg, rsync, cloud copies — is variations on the same four ideas.