Removed HDD-2 from NAS (while running) and plugged into a Sharkoon USB-Bay and tried to recover data.

Tried to assemble all existing volumes:

  % sudo mdadm --assemble --scan
  mdadm: /dev/md/9 has been started with 1 drive (out of 24).
  mdadm: /dev/md/256 has been started with 1 drive (out of 2).
  mdadm: /dev/md/13 has been started with 1 drive (out of 24).
  
  % cat /proc/mdstat
  Personalities : [raid1]
  md13 : active (auto-read-only) raid1 sdb4[1]
        458880 blocks super 1.0 [24/1] [_U______________________]
        bitmap: 1/1 pages [4KB], 65536KB chunk
  
  md256 : active (auto-read-only) raid1 sdb2[1]
        530112 blocks super 1.0 [2/1] [_U]
        bitmap: 0/1 pages [0KB], 65536KB chunk
  
  md9 : active (auto-read-only) raid1 sdb1[1]
        530048 blocks super 1.0 [24/1] [_U______________________]
        bitmap: 1/1 pages [4KB], 65536KB chunk

/dev/sdb3 did not get detected. Started raid manually:

  % sudo mdadm --verbose --assemble /dev/md1 /dev/sdb3
  mdadm: looking for devices for /dev/md1
  mdadm: /dev/sdb3 is identified as a member of /dev/md1, slot 1.
  mdadm: no uptodate device for slot 0 of /dev/md1
  mdadm: added /dev/sdb3 to /dev/md1 as 1
  mdadm: /dev/md1 assembled from 1 drive - need all 2 to start it (use --run to insist).
  % sudo mdadm --readonly --run /dev/md1
  mdadm: started array /dev/md1

Activated volume group:

  % sudo pvscan 
  % sudo vgchange -ay vg1
  2 logical volume(s) in volume group "vg1" now active

Decrypt volume:

  % perl -e 'print crypt("__PASSWORD__HERE__", "\$1\$YCCaQNAP\$")' > qnap.key
  % sudo cryptsetup open /dev/vg1/lv1 test --key-file=qnap.key

See: https://www.kormoc.com/2014/06/10/unlock-qnap-encrypted-disk/ ¹

Mount volume:

  % sudo mount -o ro /dev/mapper/test /mnt/source/

Unmount and stop volumes:

  % sudo mount -o ro /dev/mapper/test /mnt/source/
  % sudo cryptsetup close test
  % sudo vgchange -an vg1
  % sudo mdadm --stop --scan

Re-Inserted HDD in QNAP and the rebuild process was started automatically.