Erase and overwrite Disk on Synology DiskStation

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:

To identify the disk use fdisk and/or hdparm:

To run the dd command furthermore after disconnection from the SSH connection, use nohup: