I would suspect the cabling and/or the dock before all your drives.
Writing zeros to hard disk does not securely delete the contents. https://en.wikipedia.org/wiki/Gutmann_method
A further problem with writing zeros to the contents to /dev/sdf/ is that contents of any relocated sectors will not be over-written. It might be possible to recover data from those bad sectors.
If you can get the dock working, look at using hpparm to do SATA secure delete? It has flags to support this.
https://www.putorius.net/securely-erase-a-sata-hard-drive-hdparm-linux.html
You could then use hpparm to set an unique, LONG, random password on each drive.
Those machines have a fan at the bottom of the case.