Asus M3A78-EMH HDMI - Manual (Page 27)

Chapter 3: Setup
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The Select Drives screen appears.
6. If you want to split the capacity of your physical drives between two logical
drives, enter the capacity for the first logical drive in the Logical Drive Size
field.
Or, to use the maximum capacity of the physical drives, check the Use
Maximum Capacity box.
For RAID Ready and JBOD, the system will check the Use Maximum
Capacity Box automatically.
7. Click the physical drives to select them.
Available drives have a black frame. Selected drives have a red frame.
For RAID Ready, select only one physical drive.
8. Click the Next button.
The Assign a Name screen appears.