The 3-2-1 rule
- Three copies: the original and two backups.
- Two types of storage: for example a NAS and an external drive or the cloud.
- One off-site: after a fire, theft or flood, a copy survives elsewhere.
What is not a backup
- RAID protects against a disk failure, not against deleting a file.
- Syncing (Dropbox, OneDrive) will happily delete or encrypt a deleted or encrypted file in the cloud as well.
- Snapshots on the same device help quickly, but if you lose the device they disappear with it.
Rotation and immutable backups
Grandfather-father-son rotation (GFS) keeps daily, weekly and monthly versions. Immutable backups, which cannot be changed or deleted for a set time, help against ransomware.