NT4.0 add 2 drives and setup mirroring

I have an NT4.0 server that has 5 9GB drives setup in RAID 5. I also have a 4.3GB split with operating system and cache on it. This is my C drive and is setup in FAT. I have added 2 more 9GB drives that I would like to set for C drive with mirroring and have a small FAT partition along with the rest being NTFS. I tried to setup mirroring for the 2 drive in NT Disk Administrator with no luck. I have the 2 drives showing and tried to click the Fault tolerance menu option with no luck as the mirroring option was greyed out and I cannot set. Any ideas on why this is happening?