Hello Fbsd, thanks for contacting us through the Intel Communities.
First of all I suggest you backing up all the data before performing any change on your array. Since you had a RAID 1, but you reinstalled the OS on one of the hard drives, please save the information from the other hard drive.
In order to rebuild your RAID, you need to boot to Windows or to the Operating system you are using. Then, install the Intel® Rapid Storage Technology, version 11.7 https://downloadcenter.intel.com/Detail_Desc.aspx?DwnldID=22194
The following URL will guide you into the process of rebuilding your RAID1.
http://download.intel.com/support/chipsets/imsm/sb/irst_user_guide.pdf
Regards.