ASROCK 775DUAL-880PRO Manual To Installation And Configuration page 11

Create RAID 1
The data on disk drives will be destroyed if user uses "Auto
Setup" to create RAID 1. But you can reserve the data on source
drive if you use "Select Disk Drives" to select the source and
the mirror drive.
Press "y" to copy data of source to mirror drive. There is a
limitation when using this feature. The capacity of mirror drive
must be greater or equal to the source drive; otherwise the RAID
1 can't be created and a error message will appear: "Error:
mirror's size is smaller than source!!! Press ESC return". If user
does not want to duplicate data, the data on the source and the
mirror drive will be destroyed.