Create a RAID1 array of 2 disks with mdadm.
1. Partition both disks (fdisk).
2. mdadm --create /dev/md1 --level=1 --raid-devices=2 /dev/hdi1 /dev/hdj1
3. wait a long time, check progress with: cat /proc/mdstat
4. create the filesystem (ext3): mke2fs -j /dev/md1
5. add the devices and array to /etc/mdadm.conf:
DEVICE /dev/hdi1 /dev/hdj1
ARRAY /dev/md1 devices=/dev/hdi1/,/dev/hdj1
6. Don't forget to add the device to /etc/fstab