If a single drive fails from a raid array, you will need to replace the drive and restore all data from backup.

RAID Level 1

Explanation:

The RAID 1  is a category of the redundancy array of independent disks and drives. It is known as disk mirroring. It stores identical data to pair of drives. Its primary use is for redundancy, if a drive fails, the system can still access data from other drives and when the drive is replaced with another, all the copies of data is placed in the new drive, either from other disks or from a backup.

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