Try mounting them. What is the dd command you’re using?
Now, depending on your threat model maybe a drill and a sledgehammer will be enough. If your threat model is rather high and it is really sensitive data, well you’ll have to spend that money if you can’t get zeros written.
Yeah. That should do it. There is also a shred tool you could try.
sudo shred -vfz /dev/sdf
You can also write a zero file to see if the drive is useable. That won’t erase what’s on it, but it will write all free space. If you mount the drive and delete everything on it, a zero file will in theory write zeros to the whole drive.
mount /dev/sdf /media dd if=/dev/zero of=/media/zerofile.tmp
I have a similar threat model. If the disks are indeed bad then a nice sledge and a trip to an e-cycle location should do it.