Gateway ADAC Ultra2 S466, Hardware Manual

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Chapter 2 Introduction to RAID 11
Disk Mirroring
With mirroring (used in RAID 1), data written to one disk
drive is simultaneously written to another disk drive. If one
disk drive fails, the contents of the other disk drive can be
used to run the system and reconstruct the failed drive. The
primary advantage of disk mirroring is that it provides
100% data redundancy. Since the contents of the disk drive
are completely written to a second drive, it does not matter
if one of the drives fails. Both drives contain the same data
at all times. Either drive can act as the operational drive.
Disk mirroring provides 100% redundancy, but is
expensive because each drive in the system must be
duplicated.