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If you have information in both hard drives I would recommend cleaning one of them (preference disk D:), you should be able to create a RAID 1 without losing the information on disk C:.
You can find more details on how to create the RAID on User Guides for Intel® Rapid Storage Technology, Intel® Matrix Storage Manager, Intel® Matrix Storage... on the PDF: Intel® Rapid Storage Technology (Intel® RST) User Guide (PDF)
If you need to download the Intel® RST user interface you can refer to https://downloadcenter.intel.com/product/55005/Intel-Rapid-Storage-Technology-Intel-RST-, never the less it is always recommended to use the drivers provided by the motherboard manufacturer, we recommend contacting gigabyte to download the correct Intel® RST version.
NOTE: We recommend having a back up of the information before making any changes to the system.
Intel Customer Support Technician