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Hello,
Thank you for providing debug logs and remote access.
I took a look through the logs, and it appears that at the time the logs were generated the NAS already did what I would've done; it automatically force reassembled the RAID array after the last reboot, and your data is available and the volume is mounted in a read-write state.
While looking through the logs I also noticed that there is evidence of filesystem errors on your volume, too. Filesystem errors are critical problems with a volume's filesystem that can cause a wide-array of issues including broken or abnormal DSM functionality and data corruption. Filesystem errors are most commonly caused by an abrupt power loss while the device was performing a critical operation. If a volume's filesystem has issues, it will also not be able to expand. The surest way to resolve this is to backup the data on the volume, remove it, create a new volume, then restore from backups. We can also run a filesystem check, but I would advise against running any checks until the storage pool has been repaired to minimize the risk of data corruption.
Overall, my recommendation is to finish backing up your data from the volume, then remove the existing volume, replace disk 3 too, then create a fresh volume and restore from your backups. The volume should be in a stable enough state to complete the backup; looking at the logs it appears that disk 3 is still (relatively) healthy, just not healthy enough to complete a RAID repair.
Another potential alternative you could look into would be to clone disk 3 to another drive and then swap in the cloned drive before repairing the storage pool with the replacement disk 1. Do however note that we would not be able to assist in cloning any drives.
If you have any further questions, please feel free to reply to this message.
Thanks,