Just installed ugreen dxp4800 gt - keeps rebooting after 30 seconds

As it says, had the nas for a few weeks and decided to switch to truenas (nfs 4 primarily).

Added an intel m.2 128Gb: already have 2 x 8Tb spinners and 2 x 512 SSDs for fast cache.

install all appeared to behave fine, boots OK and finally pops up little menu on the console, then after about 30 secs reboots. I can access it over the network for a few seconds before it resets itself..

Cannot boot into safe mode as img file not present.

Any ideas?