For a job, I have to wipe a lot of disks that contain sensitive information. Those disks are all identical, I cannot wipe more than one at the same time, and fully wiping one takes me a full hour.
To gain some time, I though about stopping
dd if=/dev/zero of=/dev/sda whenever it wrote enough data. All those disks are 1TB of capacity, but only 30 GB were used. So if I write ~30GB (or let's say 60GB to be sure), is it safe to consider that all data were safely erased ? I don't know much about where does ext4 allocate the files on the disk.