Adding new RAID 1 disks to existing SSD in Gigabyte Dual Bios

You should not have any trouble doing this. To start, check the CD that came with your mobo for a manual that tells you about using the RAID system in your mobo. This is often separate from the mobo manual.

Basically, the sequence will be:
1. Install the 2 new HDD's in the machine and connect to regular SATA ports.
2. Boot into BIOS Setup and verify the HDD's are there and detected correctly. See your mobo manual, p. 24, "On-Chip SATA type", and set this to RAID. Then set "OnChip SATA Port4/5 Type (SATA3 4/SATA3 5 connectors)" to "As SATA type". Remember to SAVE and Exit.
3. As the reboot progresses you need to press a particular key set to enter the RAID Management software that is built into your BIOS. Often it is something like <Ctrl>-I or a Function key. Watch for the prompt that tells you the right key(s).
4. Within that system you can see all your storage devices. You can choose to establish a RAID array of a particular type. Then you get to specify exactly which devices are part of the array you're creating. By default, any device NOT assigned to a RAID array is NOT a RAID unit - it is left to function as a normal stand-alone device. So that is how your SSD will NOT be affected. Complete the setup for your 2 HDD's in this new RAID1 array. The last step is to actually Create the array, which basically is like Partitioning and Formatting the disks and can take a long time.
5. When done you exit the Management routines and the machine finishes booting normally. At this point your Windows probably will detect the existence of a new device (the RAID array) and offer to install the necessary driver(s) for it. If those are not yet on your HDD, you may have to use your mobo's CD to get them. You MAY have to reboot, maybe not. AFTER the driver is installed you will be able to see the new storage device (the RAID1 array) as a single "drive" in My Computer and use it.

You may have seen discussions of installing RAID drivers from floppy disks or USB drives as part of a complete Install of Windows. You do NOT need to do it this way - that process is only for setting up a RAID array that you must BOOT from. For your situation - boot from a different stand-alone device (the SSD) and use the RAID array only for data storage - the driver installation is done from within an already-installed and running version of Windows, as above.

In my experience (a different system), the RAID array had to be physically installed and created using the Management software BEFORE Windows could detect it and install the required driver. So that's the sequence I outlined above.

MAYBE the mobo is having trouble with boot device priority. Boot into BIOS Setup and set "On-Chip SATA Type" to RAID (See mobo manual Section 2-5, p. 24.) Now go back to Section 2-5 on p.22. Set "Boot Option Priorities" as you wish (I use optical first, then HDD). Below that is the item, "Hard Drive/CD/DVD ROM Drive/Floppy Drive/Network Device BBS Priorities". In that you go to the section for Hard Drives and specify which HDD you want to boot from. Make sure it is your SSD, and NOT either of the empty new HDD's. Back out, Save and Exit, and I would hope your machine can boot properly that way. THEN you can reboot and proceed to establish the RAID1 array.