root@Tower:/mnt/disk5# mkdir test
mkdir: cannot create directory ‘test’: No space left on device

tower-syslog-20250125-0404.zip

Attach Diagnostics to your NEXT post in this thread 

this is my tower-diagnostics-20250125-1648.zip

You should avoid filling up a COW filesystem like btrfs or zfs past 90%, post the output from:

 

btrfs fi usage -T /mnt/disk5

 

Overall:
    Device size:           3.64TiB
    Device allocated:           3.64TiB
    Device unallocated:           1.04MiB
    Device missing:             0.00B
    Device slack:             0.00B
    Used:               3.54TiB
    Free (estimated):          97.02GiB    (min: 97.02GiB)
    Free (statfs, df):          97.02GiB
    Data ratio:                  1.00
    Metadata ratio:              2.00
    Global reserve:         512.00MiB    (used: 0.00B)
    Multiple profiles:                no

              Data    Metadata System                            
Id Path       single  DUP      DUP      Unallocated Total   Slack
-- ---------- ------- -------- -------- ----------- ------- -----
 1 /dev/md5p1 3.63TiB 11.00GiB 16.00MiB     1.04MiB 3.64TiB     -
-- ---------- ------- -------- -------- ----------- ------- -----
   Total      3.63TiB  5.50GiB  8.00MiB     1.04MiB 3.64TiB 0.00B
   Used       3.53TiB  5.00GiB 96.00KiB

The filesystem is fully allocated, you won't be able to write any more data despite still showing some free space, since new metadata chunks will fail to allocate.