X-RAID2 is effectively RAID-1 if 2 disks are installed. It is effectively RAID-5 with more than two disks.
First I would suggest you update to the latest firmware (currently 4.2.20: http://www.readynas.com/RAIDiator_x86_4_2_20_Notes
). Reboot when prompted and verify the update completed successfully.
Initiate a factory default e.g. via System > Update > Factory Default in Frontview, then open RAIDar (http://www.readynas.com/downloads
). The NAS will reboot and a few checks will be run then there will be a 10 minute window in which you can click setup in RAIDar and choose RAID modes. Choose Flex-RAID and the relevant RAID level then confirm your choice.