ReadyNAS RN424 | Inactive Volume + RAID Issue

OK, so first and foremost,StephenB is an absolute legend. He took time out of his day to review the logs, and write an amazingly detailed guide for how to fix it. I'll summarize the high-level points with a disclaimer... if you're reading this, perhaps your issue is different. This is how I solved my issue, and Stephen was able to look into the logs to know this was the issue.

1. I removed the drive in slot 3 (the original one I tried to hot swap)

2. I re-booted the device in Tech Support mode (power down device, hold reset switch, power up device, keep holding reset switch, choose tech support mode)

3. I connected to the device with telnet (telnet ip-address-here) using root:infr8ntdebug

Then, I ran the following commands:

1. rnutil chroot (I think this logs me in to root user or something?)

2. mdadm --stop /dev/md127 (stop active partition)

3. mdadm --assemble --really-force /dev/md127 /dev/sda3 /dev/sdb3 /dev/sdc3

4. btrfs device scan

5. mount -r ro /dev/md127 /data

At this point, I had a working /data partition (huzzah) that I could inspect with ls (e.g. ls /data) and which showed the data I'd expect, as well as additional data for the Netgear device.

Finally, I BACKED UP the critical data just to avoid this issue going forward -- and also set up an rclone while I was at it to keep it up-to-date -- and then I inserted the new drive into slot 3 which is now being recovered.

A journey, but the destination was beautiful. Thank you again to StephenB, you are a lifesaver, and a saint.