Just ordered a new H1110 LSI 9211-4i card with a new SAS-to-SATA breakout cable. I'm really hoping this does the trick because I'd hate for the problem to be the backplane of the case.

I decided to take another shot at remote troubleshooting today with a different family member and we found out that one of the Molex power connections had come loose on the backplane, where it was very slightly loose that it made imperfect contact and _appeared_ to power the drives. For context, the backplane has two Molex plugs that splits off into the five disks.

 

I'm not sure how the Jonsbo N1 has the two connectors routed on the backplane PCB, but I believe that it was trying to draw all the current from one Molex connector, and because of the voltage sag it was manifesting as disks dropping off of the array. I am not sure why or how the failures followed the two drives, but perhaps they are the most susceptible to the voltage sag?

 

 

After firmly seating the Molex connectors and rechecking all connections the server was turned back on and the two new drives immediately showed up. Currently running a parity-swap procedure and it is going well now for half an hour. Previously even a simple parity check would fail after 30 seconds. Knock on wood and all that but I think this problem has been actually fixed this time.

 

If we checked the connections properly it would've saved all the headache (not to mention the cost of the new drives... at least now I have two cold spares?) But troubleshooting remotely with family has limitations and I can't exactly tell if connections are firmly pressed in over a video call and have to take the other person's word for it. So unfortunate but glad it was resolved in the end.

 

Thank you @JorgeB for the troubleshooting help! I've marked your initial reply as the solution because it was indeed a connection problem.