fstab - making changes persist

For a long time my Firewalla Purple worked great with the 512 GB SD card I keep in it.  After one of the updates some months ago something changed and now the Purple wont boot with the SD installed when it's powered up. 

I am pretty sure it's due to /etc/fstab not maintaining the updates I make assigning the SD card to a mount location.  When I change /etc/fstab the changes are lost after a power cycle.

Where can I make changes to fstab that will persist through reboots and power cycles?