stor44

UPDATE: I was able to solve the problem. I used the unRAID USB Creator app on macOS to create a new USB stick using the zipped backup from my old stick, as per these instuctions:
Changing the flash device | Unraid Docs
The USB flash device is critical to your Unraid server as it stores the operating system, configuration files, and license. If this device fails or becomes unreliable, prompt replacement is essential
I shutdown my unRAID server, swapped the failed USB stick with this new one and booted it up. I set the password, then confirmed the drive assignments were still correct and started the array.
Seems back to normal now.

Step 3 worked for me, using Google Chrome browser, thanks! I had made a fresh unRAID USB stick for a new server and couldn't register it in the Connect plugin for the life of me. Up and running now.

Update. I'm up and running with the new power supply and have completed a parity-sync rebuilding Drive 3 to itself, and things seem back to normal.
I found a solution in ASUS's help pages here, in Step 1 C. Turns out my motherboard has 3 power connections. The 24 pin, an 8 pin and an extra 4 pin. I didn't have the 4 pin connected! Fortunately the new PSU came with a two cables that have 8 pin connectors which split into 2 x 4 pins on one end. The AX860i does not seem to have those, but maybe I lost them at some point.
The parity sync completed without errors, and hopefully I've seen my last Red X for a while.
Thanks as always for your patience and advice, I really appreciate it.