As for the power supply, the only time you will have issues is if/when all the drives spin up at exactly the same time.
Is a way to make sure the spin up on the drives is staggered, and what type of drives you are using.
Major spin up load is on the 12v rails, and HGST run about 2A Peak (if I remember correctly-just went though a similar design exercise), so you have 26A surge on the the 12v rail of your PS. 26x12=312W
If the drives are running full out about 10W (7W is more likely) is a a very generous maximum - so 8x13=130W
Say another 130W for RAM/CPU, and 30W for fans that's just about spot on for the sweet spot of a 500W supply (55-60%).
Get the data sheets on the drives and add up the maximum current draws and compare it to the power supply rating, then add in an estimate for the MB/CPU/RAM and whatever else is in the case.
If you have a good quality supply you are likely OK, but if you have an elCheap, it's likely trouble. If it's a name brand see if there has been any bench testing done on your unit (sites like
https://www.hardocp.com have some good info). Pay attention to transient response tests, overload tests, and what the quality of the design/parts are.
I've got a Seasonic 560W and the 12V rail has a maximum rating of 46A-it would handle that load very well-especially if the machine runs 24x7 and isn't powered off very often.
As for ECC, have a look at these:
https://youtu.be/vxFNBZIAClc
https://youtu.be/52x4PSxbjUg
I can also say from personal experience in the 4 years I have been running FreeNAS I likely had a couple of memory problems for about 2 of that, and never lost any data (I'm sure a bit of luck goes with that), but ECC is certainly a good idea, but it's definitely not connected with the issue you are reporting.
