American Megatrends MegaRAID Express 500 - Manual (Page 25)

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.