Your main problem is how to BOOT from a disk drive. With Win 8 installed, it has its own drivers for IDE and SATA drive types, so it uses your HDD just fine now. BUT it does NOT have any driver for a RAID array. In fact, since RAID is done differently by many suppliers, there is no universal RAID driver. Net result is that, if you bought another identical HDD, installed it, and used the RAID management system in your BIOS to convert your single HDD into a RAID array, you could NOT boot up from it!!
The way this is supposed to be done is: when you are first installing Win 8 from the CD, one of the options offered is whether you want to install any special device drivers (like a RAID driver) from some external data storage device. It used to be that device had to be a floppy disk, but I think Win 8 will let you install it from a USB memory stick. Check the Win 8 documentation for what devices can be used for this driver install process. NOTE that this means you will need to re-do your entire Install process, thus losing all your old data. SO, that means you will need to make a complete BACKUP of your current HDD, then VERIFY that the backup is good and you can get the data back from it.
The steps:
1. Buy and install a new HDD identical to the one you have.
2. Install (even if only temporarily for this process) some other storage medium. Make your backup on this, then verify it is good. Disconnect this unit until you need it again to restore from it.
3. Find and READ the manual for the RAID system built into your mobo. Often it is a separate file on the CD that came with your mobo.
4. In your mobo manual, you should be able to find a section detailing where to find the RAID driver for your system. Often it's on the CD of utilities that came with the mobo. You will need to make a copy of this driver onto some medium (like a USB stick) that you can use later during the Install process.
5. When you're all prepared, place your Win 8 Install CD in your optical drive and boot up. Go IMMEDIATELY into BIOS Setup and set it so that the two HDD's are in RAID mode. Check the Boot Priority Sequence and ensure it will try to boot from your optical drive first. Save and Exit, and WATCH for the prompt about how to enter the RAID Setup system, and do that. In there, according to the RAID manual, configure those two HDD's to be one RAID1 array that you boot from. You might have to let it delete the existing Partition on your one older HDD. You will have to let it Partition and Format them, which takes time.
6. After the RAID array is created, you can let it continue and boot from the Win 8 Install CD. Watch for the prompt that asks whether you wish to install any additional drivers, and you must press the "F10" key to do that. If you do not, it will wait a bit and proceed to the next steps.
7. When you do press F10 there, it will prompt you to tell it which driver you want to install from what storage device. Do that, and it will install that RAID driver, customized for your mobo, that you copied to your storage device. This will become a part of the basic Win 8 Install in this machine, allowing you to boot from that device (the RAID1 array). This will also give the Install process a way to access that RAID1 array you created as a "drive" it can install Win 8 on.
8. Proceed with the rest of the installation. When you're done, the RAID1 array will appear to be one "drive" called C:, with a capacity of ONE of your two HDD's - that is, 1TB (more likely, about 930 GB) because the second HDD is a complete mirror copy of the first.
9. When it's all done, you can start to re-install all your application software - things like Microsoft Office, Firefox, Adobe Reader, an internet security suite, games, graphics software - whatever. You MAY run into snags when your software thinks you are trying to do a second (unauthorized) installation on a new machine, and that will require some discussion and help from the software maker.
10. Once all your software is installed and it has created its preferred folders for files, shut down and re-install that backup device. Restore all your user data files from it to your C: drive. When done, disconnect the backup device and keep it safe for a few months. If you later find you missed restoring something, you can hook it back up and look for it on that device.
NOTE: you can NOT simply Clone an image of your current HDD to another, and then clone that image back after creating the RAID 1 array. The Win 8 installed on your current HDD does NOT have the special RAID driver installed, so you would not be able to boot from that restored image. YOU MUST do the re-install of Win 8 with the addition of the driver. That also means you MUST re-install all your application software on that newly-installed Win 8 system.
Now, a clone of your current HDD IS a useful backup form. It will contain everything, AND a typical clone made by Acronis True Image is NOT encoded in any way, so it is completely readable by your newly-installed Win 8. Thus you CAN use it as a way to copy all your user files back to your new C: drive. It's just not a simple a re-cloning back.