IBM ServeRAID-4 Ultra160 Series User Reference page 36

The data is striped across the drives, creating blocks.
Notice that the data on the drive on the right is a copy
of the drive on the left.
With RAID level-1, if one of the physical drives fails, the ServeRAID controller
switches read and write requests to the remaining functional drive in the RAID
level-1 array.
Understanding RAID level-1 Enhanced
If you have three or more physical drives in the array, you cannot select RAID
level-1; you can select RAID level-1 Enhanced (RAID level-1E).
RAID level-1E combines mirroring with data striping. This RAID level stripes data
and copies of the data across all of the drives in the array. As with the standard
RAID level-1, the data is mirrored, and the capacity of the logical drive is 50 percent
of the array capacity of the grouping of physical drives in the array.
RAID level-1E requires a minimum of three drives and, depending upon the level of
firmware and the stripe-unit size, supports a maximum of 8 or 16 drives.
The following illustration is an example of a RAID level-1E logical drive.
Start with three physical drives.
Create an array using the physical drives.
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IBM User's Reference: ServeRAID
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-4 Ultra160 SCSI Controller
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