Windows: Format and write zeros to every sector of the drive.
format <Driveletter> /fs:NTFS /p:0
Windows: Format and write zeros to every sector of the drive. After the first pass write random numbers.
format <Driveletter> /fs:NTFS /p:2
Windows: Fill free space.
cipher /w:<Driveletter>[:\foldername]</span>
Unix: Wipe full drive.
dd if=/dev/zero of=/dev/<block device> bs=<block size 1M, 32M, etc.> status=progress
Unix: Fill free space.
dd if=/dev/zero of=/path/to/drive/zeros.file status=progress
Unix: Fill free space on FAT32 drive (4GB limit per file).
cat /dev/zero | split -b 2000m - zero -d --additional-suffix=.file
To run the process in background see Run command in background on a Synology NAS with nohup