RAID is not a backup: Protection against failure versus protection against loss

In short: RAID keeps a system running when a hard drive fails but does not protect against deletion, viruses, fire or theft; for that you need a real backup in another place.

In more detail: A common misconception: “My data is on two disks, so it is safe.” RAID (redundant array of independent disks) distributes or mirrors data across several drives to increase availability. Backup creates an independent, older copy to restore data.

In Depth

What RAID achieves

• RAID 1 (mirroring): two disks with identical content; if one fails the system keeps running.
• RAID 5/6: data plus parity over 3+ disks; one (RAID 5) or two (RAID 6) disks may fail.
• RAID 10: mirroring and striping, fast and safe.
• RAID 0 (striping): speed only, no redundancy – one disk gone, everything gone. RAID protects against hardware failure of single drives and allows repair during operation.

What RAID cannot do

• Accidental deletion or overwriting: executed immediately on all disks.
• Ransomware, viruses, software errors, file-system errors: are mirrored too.
• Fire, water, lightning, theft: affect all disks in the same device.
• Controller or power-supply faults that destroy the whole array.
• Several simultaneous failures (same batch, high load during rebuild).
• User errors during replacement/rebuild.

Backing up properly

Use the 3-2-1 rule: 3 copies, on 2 different media, 1 in another place (cloud, second site). Add versioned backups (snapshots, Time Machine, restic), an offline/offsite copy against ransomware and restore tests. RAID/NAS is part of operation, the backup lies additionally outside. Cloud synchronisation is not a backup either (deletions are synchronised).

When is RAID worthwhile?

When downtime is expensive (servers, NAS for family/office, video editing with constant access). For a private system one disk plus a good backup often suffices. Plan spare drives, monitoring (SMART, email alarm) and the restore. When building use drives of different batches.

See also: RAID, Backup strategy (3-2-1), NAS server, Drive and SSD health