Also just to update some folks on this type of thing...
Because freenas would be controlling your raid, in the BIOS you will always see it listed as separate drives. Same for many DIY NAS units, like unraid, etc.
The onboard RAID on intel motherboards (available on most consumer PC's) stores the config in BIOS. So, if the battery dies while the computer if unplugged, or the mobo gets fried, etc, RAID config gone.
Older RAID cards also worked like this (Esp Dell and IBM) storing the info on flash on the card. If the card died, the array was poof. Hence why many older servers used redundant RAID cards. Funny, using redundant array cards to protect your Redundant Array of Independent Disks.
Almost all new "server grade" RAID cards store the information for the entire RAID set on each disk drive in the array. Hence how you can move drives around and everything still works. Note, I do NOT recommend that.
Finally, a reminder - RAID do NOT equal backups. Even with modern hardware, it it easy for a raid card to fry and wipe all your data. I have seen it, Linus has seen it.
Don't forget to backup that critical data.