So first off, the TOS software is junk. I came from a synology, so I knew that it wouldn't be great, but the lag and just general dashboard failure was too much for me. I honestly do not trust it to protect your data. Enter freenas.
First thing is to remove the USB stick mounted INSIDE the terrmaster. It is a little USB stick that is glued to the header under the drive bay. If you do not do this, it will always try to boot it. If you want to use this space, you might need a very skinny USB like a data traveler. You could rewrite over their USB, but I chose not to do this so I can eventually sell this hardware 10 years from now with their OS on it.
So get your new stick, and use unetbooten or rufus to make a bootable USB with the freenas ISO from their downloads. For some reason when I tried to make the bootable USB with OSX, it would just fail a lot. I ended up using rufus in windows and it worked fine. Insert it into the powered off terramaster.
Now the bios in the terramaster is AMI. Start tapping the del key and you should get into the bios. While you're in here set your hardware fan control to max. This is because unraid and freenas have trouble seeing hardware fan control. To err on the side of caution, make sure this is on or you could cook this thing eventually. Now go to the boot order and make "USB" if you are using inside header. Usually can just see whatever you USB drive name is. Set this boot order before the HDs so that if this thing ever reboots, it goes right into freenas and not your HD that has no OS. F10 and leave.
As this thing boots up start hitting F11. Select the named USB drive you want. I ended up using legacy name of the USB, when I chose UEFI nothing happened. Now you should see the freenas splash to start your install.
NOTE: You need an additional USB plugged in to install freenas to. So ideally, already put the USB you want it to run off inside the terramaster under the cage on the motherboard. That way you always have a slot free on the back.
As for using the OS you're on your own. It's definately not the most user friendly, but there are so many awesome features it's worth it. I really only use SMB shares and a timemachine on AFP. I personally do not need all the neat synology looking front end. I can connect with my appletv to a video folder and play something and that's enough for me!
Good luck!
