These two videos show how to set up a software RAID1 under FreeNAS.

If you’re wondering what RAID1 means, RAID1 uses mirroring to write data to for instance two drives. This means that when you write a file or save a video, the file is written to two disks. If one of the disks fails, you simply replace it and rebuild the mirror, i.e. one disk is an exact copy of they other. The tradeoff with this setup is cost. With RAID1, you purchase double the amount of storage space that your data requires.

Video 1

Video 2