External drives · Choosing a drive
What is a RAID enclosure, and does it replace a backup?
Short answer: A RAID enclosure combines several drives: striped for speed and size (one failure loses everything) or mirrored for redundancy. A mirror is not a backup — deletions and damage copy to both drives — so keep a separate backup either way.
A RAID enclosure combines two or more drives into one. Striped sets are faster and bigger but lose everything if any one drive fails; mirrored sets keep identical copies so one drive can fail without losing data. macOS can also build simple sets with Disk Utility → File → RAID Assistant.
A mirror is not a backup: deleting a file, a mistake or a damaged file is copied to both drives instantly. You still need a separate backup on another drive or in the cloud.
For most people, one good SSD plus a backup is simpler and safer than RAID — see what kind of drive to use.