Software RAID on 2.2.x kernels require a patch. I never really got root raid to work nicely until 2.4.x
For a daily backup to that drive, I would just use the following (stolen from the "Root RAID How-To") commands. This stuff doesn't require RAID to run, it's just a handy tool for copying lots of stuff.
First you need to partition and format the new (empty) drive. Please make sure it's the right one! Then mount it to something like /mnt
find . -xdev | cpio -pm /mnt
You could use this in a script and mount/copy/unmount every day.