No space left on device but only 50% space and 1% inodes used

This is on a Iomega IX200 NAS which had been expanded to 4TB disks from the original 2TB. It all looks good. But when I try to save data to a new file I get the "No space left on device error". I get this whether I try to save a file via the NAS drive share on another device or vi the SSH session on the NAS itself. df -h reports:

Filesystem            Size  Used Avail Use% Mounted on
rootfs                 50M  3.7M   47M   8% /
/dev/root.old         6.5M  2.1M  4.4M  33% /initrd
none                   50M  3.7M   47M   8% /
/dev/md0_vg/BFDlv     4.0G  624M  3.2G  17% /boot
/dev/loop0            592M  538M   54M  91% /mnt/apps
/dev/loop1            4.9M  2.2M  2.5M  48% /etc
/dev/loop2            260K  260K     0 100% /oem
tmpfs                 122M     0  122M   0% /mnt/apps/lib/init/rw
tmpfs                 122M     0  122M   0% /dev/shm
/dev/mapper/md0_vg-vol1
                       16G  1.5G   15G  10% /mnt/system
/dev/mapper/5244dd0f_vg-lv58141b0d
                      3.7T  2.0T  1.7T  55% /mnt/pools/A/A0

/mnt/pools/A/A0 is the one that provisions the storage. df -h -i:

Filesystem            Inodes   IUsed   IFree IUse% Mounted on
rootfs                   31K     567     30K    2% /
/dev/root.old           1.7K     130    1.6K    8% /initrd
none                     31K     567     30K    2% /
/dev/md0_vg/BFDlv       256K      20    256K    1% /boot
/dev/loop0               25K     25K      11  100% /mnt/apps
/dev/loop1              1.3K    1.1K     139   89% /etc
/dev/loop2                21      21       0  100% /oem
tmpfs                    31K       4     31K    1% /mnt/apps/lib/init/rw
tmpfs                    31K       1     31K    1% /dev/shm
/dev/mapper/md0_vg-vol1
                         17M    9.7K     16M    1% /mnt/system
/dev/mapper/5244dd0f_vg-lv58141b0d
                        742M    2.6M    739M    1% /mnt/pools/A/A0

When the partition was grown I ran lvresize and xfs_grow, after which it started showing as having 3.7Tb capacity. Disks/partitions:

$ parted -l

Model: Seagate ST4000VN008-2DR1 (scsi)
Disk /dev/sda: 4001GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt

Number  Start   End     Size    File system  Name     Flags
 1      36.9kB  21.5GB  21.5GB               primary       
 2      21.5GB  4001GB  3979GB               primary       


Model: Seagate ST4000VN008-2DR1 (scsi)
Disk /dev/sdb: 4001GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt

Number  Start   End     Size    File system  Name     Flags
 1      36.9kB  21.5GB  21.5GB               primary       
 2      21.5GB  4001GB  3979GB               primary       


Model: Linux device-mapper (linear) (dm)
Disk /dev/mapper/5244dd0f_vg-lv58141b0d: 3979GB
Sector size (logical/physical): 512B/512B
Partition Table: loop

Number  Start  End     Size    File system  Flags
 1      0.00B  3979GB  3979GB  xfs               


Model: Linux device-mapper (linear) (dm)
Disk /dev/mapper/md0_vg-vol1: 17.2GB
Sector size (logical/physical): 512B/512B
Partition Table: loop

Number  Start  End     Size    File system  Flags
 1      0.00B  17.2GB  17.2GB  xfs               


Model: Linux device-mapper (linear) (dm)
Disk /dev/mapper/md0_vg-BFDlv: 4295MB
Sector size (logical/physical): 512B/512B
Partition Table: loop

Number  Start  End     Size    File system  Flags
 1      0.00B  4295MB  4295MB  ext2              


Error: /dev/mtdblock0: unrecognised disk label                            

Error: /dev/mtdblock1: unrecognised disk label                            

Error: /dev/mtdblock2: unrecognised disk label                            

Error: /dev/mtdblock3: unrecognised disk label                            

Error: /dev/md0: unrecognised disk label                                  

Error: /dev/md1: unrecognised disk label

When I ran mdadm --detail I noticed the second partition of the RAID1 pair was set to 'removed':

mdadm --detail /dev/md1
/dev/md1:
        Version : 01.00
  Creation Time : Mon Mar  7 08:45:49 2011
     Raid Level : raid1
     Array Size : 3886037488 (3706.01 GiB 3979.30 GB)
  Used Dev Size : 7772074976 (7412.03 GiB 7958.60 GB)
   Raid Devices : 2
  Total Devices : 1
Preferred Minor : 1
    Persistence : Superblock is persistent

    Update Time : Thu Jan 23 03:29:04 2025
          State : clean, degraded
 Active Devices : 1
Working Devices : 1
 Failed Devices : 0
  Spare Devices : 0

           Name : ix2-200-DC386F:1
           UUID : 8a192f2c:9829df88:a6961d81:20478f62
         Events : 365631

    Number   Major   Minor   RaidDevice State
       0       0        0        0      removed   #### <<<< THIS ONE is /dev/sdb2
       2       8        2        1      active sync   /dev/sda2

When I do an examine on /dev/sdb2 I get:

$ mdadm --examine /dev/sdb2
mdadm: No md superblock detected on /dev/sdb2.

I was wondering if there is still, within the firmware, a limit (the original sized disks had a capacity of 1.8 Tb). But am now thinking the 'removed' disk is the problem, but would that explain why I can only use 1.7Tb of a 3.4Tb filesystem? /dev/sda2 reports as 'clean'. Edit: I tried mdadm --zero-superblock /dev/sdb2 Followed by mdadm --manage /dev/md1 --add /dev/sdb2 But I got the error mdadm: add new device failed for /dev/sdb2 as 3: Invalid argument.