Asus A8N-SLI Deluxe - Problem with NVidia Raid

I have the Ausus A8N-SLI Motherboard and have been using the SIL-3114
Raid connection for my 2nd hard drive (pair of Seagate ST3250824AS in
"mirrored" mode) and it worked fine.

If I try to use the NVIDIA Raid controller in "Mirrored" mode with my
matched 250Gig drives then it takes 6 hours to format it in Windows XP
and the HD Tune test program says it's really slow (2.5Mb/sec) whereas
before with the SIL-3114 controller, I was getting 55Mb/sec.

What am I doing wrong. Also can I use the NVRAID controller without
the RAID feature (2 seperate drives).