I wanted to erase and overwrite the data on some hard disks that I need to downgrade my RAID 5 from three 4TB WD Red to only two. The erase function on the webgui doesn't work - something message like "don't possible with your drive". I search and found an alternative:
dd if=/dev/zero of=/dev/sdX bs=1M conv=noerror
To identify the disk use fdisk and/or hdparm:
fdisk -l
hdparm -I /dev/sdX
To run the dd command furthermore after disconnection from the SSH connection, use nohup:
nohup dd if=/dev/zero of=/dev/sdX bs=1M conv=noerror