Power isn’t problem. Power supply, if marked correctly, should provide 40W. Board and antennas is under 8W (it was somewhere on this forum) and one Wd red should be under 4W.
I also tried to power discs by separate atx power supply and the problem remained.
Overheating most probability isn’t the problem eather. I have fan connected and also tried to hold hand on sata card chips and those didn’t even get warm enough to be uncomfortable and array failed.
Mdadm with two old wd velociraptors built ok.
Wd red + seagate nas both 4TB failed. Array with those disks build in pc than put in turris reassembled ok. Copied about 1Tb of data to it over network and it didn’t failed. Now it runs 3 days from last reboot without any problem althoughI only copied some data and watched movies over dlna.
More on raid problems in this thread SATA HDD issues.