disk mirroring raid level 1

I am trying to mirror a nt4 sp6a BDC. I log on as Administrator and use disk admin. I have a 4096kb Boot/system partition on drive 0. drive 1 has same size partition. I select the first partition on drive 0 and hold ctrl and selct drive 1 partition but the option to mirror is dithered out. I understand that to mirror the second(mirror drive must be the same size or larger). what do I need to mirror?