I recently assembled a computer which utilized the G45/ ICH10R chipset. In playing around with the RAID-1 capabilities, I noticed that it doesn't seem possible to create a new RAID-1 array utilizing one of the 2 disk drives as the source drive. Yet, the Intel RAID BIOS software insists on reinitializing both drives when you define a new array. Other RAID controllers that I have worked with, in addition to initializing both drives, also will allow you create a mirrored array using the data from one of the two drives.
The ability to create a mirrored array from an existing drive comes in very handy at times. For example, if I either need to replace the motherboard or move an array from another system.
You can migrate your data from a source drive onto a new RAID 1 volume (or any other RAID volume). This is done via the Intel(R) Matrix Storage Console (the user interface that you use within the operating system), not the option ROM. If you're in the user interface, go under Actions (if you don't see an Actions menu, click View -> Advanced), and click the option that says something like Create RAID Volume from Existing Hard Drive. It will ask you for a source drive.
Hope this helps.