Simple definition
RAID 1 mirrors data across at least two disks. Writes are replicated to mirror members so that a copy can remain accessible when a disk fails.
Technical operation
With two equal disks, usable capacity is generally the capacity of one disk. A typical mirror tolerates one member failure, depending on the remaining disk and the hardware or software implementation.
After a member fails, the array runs degraded. Replacing it starts a rebuild, during which performance and the risk of another disk failure need close monitoring.
Practical example
For example, two 1 TB SSDs in RAID 1 usually deliver about 1 TB usable capacity. If one SSD fails, the service may continue using the other as long as the controller and remaining components operate correctly.
Best practices and limitations
RAID 1 does not protect against accidental deletion, ransomware, mirrored corruption, or site disasters. Monitor disk health, test recovery procedures, and maintain backups outside the RAID array.