Reboot into the RAID BIOS of your motherboard, and delete the RAID array.
Your motherboard manual will have instructions on how to do this.
Backup any needed data because deleting the RAID array will destroy the data on both drives.
If you don’t want to delete the RAID array then you can also just go into your motherboard BIOS and change the SATA controller mode from RAID mode to IDE or AHCI mode. Save your settings and then reboot and do a fresh install of Windows on one of the drives.
Once you have Windows up and running you can then reformat the other drive.