RAID level-1 provides 100% data redundancy and requires two hard disk drives.
With RAID level-1, the first half of a stripe is the original data; the second half of a stripe is a mirror (copy) of the data, but written to another drive.
Because the data is mirrored, the capacity of the logical drive when assigned RAID level-1 is 50% of the physical capacity of the two hard disk drives in the array.
The following illustration shows data arranged in an array of two hard disk drives defined as one RAID level-1 logical drive.
Notice that the data on Drive B is a mirror copy of the data on Drive A.