Backup on external USB disk with rsync dies

I connect an USB 3.0 external disk (WD Elements 4TB) to my NAS WDMycloudEX4 (4 disks of 1TB are 2 TB) to do a NAS backup…

I access via ssh from my linux box with
ssh -oHostKeyAlgorithms=+ssh-dss root@192.168.1.3
(Where 192.168.1.3 is the local IPaddress of the NAS)

The HD (formatted as NTFS) is automounted in /mnt/USB.
then cd /mnt/HD/HD_a2 and mkdir -p /mnt/USB/USB2_c1/NAS_BKP/cartella_mune/
and finally
HD_a2 # ionice -c 2 -n 15 rsync -arv cartella_mune/ /mnt/USB/USB2_c 1/NAS_BKP/

it works crunching files for a long looong while but after all that time the NAS hangs and it requires to be unplugged. Before being totally unresponsive on the ssh shell it says that the process has been ‘killed’.

The firmware is the latest on the WD’s site: 2.12.127 .

I used ionice believing that the fault was due to too much disk usage, but the result ends up to be the same: system totally locks up.