MSI P45 Platinum Series Instruction Manual page 107

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Important
Since you want to create two volumes (Intel Matrix RAID Technology), this
default size (maximum) needs to be reduced. Type in a new size for the first
volume. As an example: if you want the first volume to span the first half of the
two disks, re-type the size to be half of what is shown by default. The second
volume, when created, will automatically span the remainder of two hard
drives.
6. Then the following screen appears for you to confirm if you are sure to
create the RAID volume. Press <Y> to continue.
7. Then the following screen appears to indicate that the creation is finished.
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