Apple RAID Utility - Manual (Page 7)

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8 Under Options, choose whether you want to use space from the additional drives to
create a second volume or leave the extra space free for creating other volumes later.
Create new volume using added capacity: Creates a RAID-based duplicate of the original
startup volume and a second RAID volume with all the remaining space available on
the underlying RAID set. You get two volumes in the Finder—a startup volume and an
empty data volume.
If you don’t select this option, RAID Utility creates a single RAID-based duplicate of the
original startup volume and leaves the remaining space on the RAID set free for you to
use to create other volumes when you want.
9 Click Migrate.
10 When the migration process is finished, quit RAID Utility.
11 Quit the Installer and restart the computer.