RN104 data-0 inactive since April, still stuck after PSU fix - need assemble advice

Hi all,

I have a ReadyNAS 104 (firmware 6.10.10) with a 4-bay X-RAID2 setup: two Seagate IronWolf 4TB drives and two WD Red 3TB drives. The 3TB baseline across all four disks forms a RAID5 volume called data-0 (8.17 TB), and the extra capacity on the two 4TB drives forms a separate RAID1 volume (931 GB).

What happened, as best I can reconstruct from the logs:

• On April 27, 2026 at 11:01 AM, the system log shows "Disk in channel 4 (Internal) changed state from ONLINE to FAILED." That drive is one of the two WD Reds. SMART data shows a burst of about 878 read errors on that drive's data-0 partition earlier that same morning, right at the tail end of the partition. No reallocated, pending, or uncorrectable sectors before, during, or since, on any drive.
• - Alert emails have apparently never worked on this box ("Alert message failed to send" recurs in the logs going back to 2016), so nobody knew about the April failure. The volume was presumably running degraded on 3 of 4 drives from that point on.
• - On September 13, 2026, the external power adapter started failing, causing all four drives to drop and reconnect erratically, which pushed the NAS into booting its factory-default recovery image.
• - I replaced the power adapter (correct NU90-9120700-I2, 12V/7A). The NAS now boots its normal OS again, all four physical drives are detected correctly, and a separate small mirror volume auto-repaired and finished resyncing on its own.
• - data-0, the 8.17 TB volume with almost all my data, has never been auto-assembled by mdadm across two separate boots since the PSU was fixed, even though the smaller pools repair themselves automatically. The admin page shows "Remove inactive volumes to use the disk. Disk #3,4."

What I've verified so far:

• All four drives pass SMART self-tests clean, zero reallocated, pending, or uncorrectable sectors.
• - I have not clicked "Remove inactive volumes," Format, New Volume, or X-RAID anywhere.
• - SSH itself fails to start on this box right now (error code 15002030001, and "Connection refused" on an actual attempt).
• - I have full log bundles from before and after the PSU replacement if useful.

What I'm hoping for:

I found a very similar solved thread here on this forum (RN104, disk kicked as non-fresh after a power problem, fixed via tech support mode and mdadm --assemble --really-force --readonly). Before I touch anything, I'd appreciate help confirming whether that's the right first step for my logs, and the correct device list for my array specifically. Since SSH won't start, I understand the path in is tech support mode via the boot menu and Telnet, unless there's a better route.

Thanks so much in advance.

Link to that similar solved thread: https://community.netgear.com/discussions/readynas-use/readynas-104---cannot-access-data---remove-inactive-volumes-to-use-the-disk/2454089