There are two solutions how to add a SSD disk to a Turris Omnia board.
I tried to use m.2 SSD with an adapter m.2-miniPCI, but the Omnia board didn't recognize it.
This solution allows using up to 2 SATA disks.
Power Supply Cable for SATA disks can be connected to the Turris Omnia board and it's not a standard one. It consists of five wires.
Power cable pinout on the board
Cable discussions in Turris Forum:
Power Supply Connector on Turris Omnia board.