Help needed RAID1 Volume

Hello, i am new here and have a problem which i hope someone can help with !!

I have a Dimension E520 which i recently upgraded to Windows 7. I then installed a new 3TB HDD as my second (non booting) drive. Then discovered it only showed 746 GB. After researching, i installed Intels Rapid Storage Technolgy and did some configuring - without really knowing what !. Now my problem begins. I now have both my HDDs showing as a RAID1 Volume, and only one HDD is showing in my explorer.

What can i do to get both HDDs showing seperately again ? If so is this done in the BIOS ?

Or do i have to completey reinstall my system ?

Any help greatly appreciated - if possible in as non technical terms as i am no computer scientist  !

Many thanks in advance.