I have tried mounting it in fstab, and exporting it via NFS in /etc/exports, but somehow fstab gets overwritten when I reboot.

Anything outside of /boot and /mnt/disk? is in RAM and recreated on reboot. If you have it working until you reboot, then any commands you run to mount and share it will need to be put into the go file so that they will run on reboot. I'm not sure putting the mount point on an existing disk is a good idea, you might want to create the mount point directory /mnt/mycustom/something as part of your script.

It must be RFS, since that's the only filesystem unraid 5.x supports for data drives.

You will also have to take care of unmounting it when the array is stopped.

Electric, were you able to get this working?

I managed to get it working with a 50gb ext3 file on the btrfs cache drive.

 

I am not sure it will make it through a reboot though.

 

The information from here and this thread https://www.ipcamtalk.com/showthread.php/54-Solving-some-NFS-issues-on-Hikvision-cameras/page10 made it possible.