mirrored RAID set
A Mirrored Redundant Array of Independent Disks, or mirrored RAID set, can help protect your data against disk failure. Your data is written on two or more disks at once, so if one of the disks fails or is disconnected, your Mac can still access the data from the other disks. If you reconnect a disk you disconnected, the RAID set can rebuild it in the background so that it contains the latest data.
You can add spare disks, also called “hot spares,” to the set, which aren’t written to unless another disk fails or is disconnected. If a disk fails or is disconnected, the RAID set rebuilds one of the spares, so it contains the latest data. If you reconnect a disk after the RAID set replaces it with a spare, the reconnected disk is treated as a spare.
You can choose to have the RAID set rebuild a spare disk automatically, or you can rebuild it yourself.
Even if you have a mirrored RAID set, you still need to back up your data regularly. Mirroring protects you from some types of hardware failure, but not from user errors or software corruption. If you delete a file, it’s deleted from the mirror as well. If software corrupts a file, it’s also corrupted on the mirror.