GUIDE深度 · Deep dive
The 3-2-1 Backup Rule
3-2-1 means at least three copies, on two different media, with one off-site. A single external drive is not a complete backup; ransomware, fire or theft can take both source and destination.
How to apply it
Local machine, external drive and cloud/off-site drive.
Rotate external drives and do not leave one connected all the time.
Restore test
Restore a random file or folder quarterly.
Record backup time, destination and success.
FAQ
Is cloud sync a backup?
Not fully; sync propagates deletions/encryption, so use version history or independent backups.
How often?
By change rate; daily for important files, weekly for the rest.