Buffalo DriveStation Duo User Manual page 6

Once launched, the RAID Utility will briefly scan for your DriveStation
Duo. This may take a few seconds.
The initial screen will show your configuration. If your Duo is new, it will
probably be in Normal mode, with two equally sized independent disks.
To change the RAID mode, click RAID Setup and choose Change Disk
Structure.
Double-click the RaidSetting icon on your desktop.
The program will open.

Using the RAID Utility