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I prefer shred for erasing magnetic drives. dd can work too, but its options are arcane enough that it's easy to make mistakes that lead to weird behavior.
If that doesn't fix the unexpected size problem, I would suspect the USB bridge in your dock. Those things are notoriously buggy.
Connecting directly with SATA is a more reliable approach. It also lets you use hdparm to tell the drive to run a secure erase cycle on itself.
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.
You could just take the platters out. I use mine a fancy coasters.
Could be a bad dock or usb controller, try a different one. Otherwise just snap the sata connector off, and most people will not bother to get anything off.
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.
I am a private person without any state or corporate secrets in hand. I do not do online banking.
My threat model I believe is limited to random drive-by actors.
I was hoping to be able to provide these drives to others to use, the screwdriver and hammer will render them into E-waste. But on that issue, once the platter assembly is disassembled and the platers are separated and mixed up the data on them is probably not recoverable? With the given that each drive has multiple platters.
A few gigs of zeroes will prevent random drive-bys. At that point the partition and filesystem table of at least the first partition is overwritten and you "can" recover files off it but you'll be missing filenames and at least half the files will be corrupt due to fragmentation losing track of which files are where.
I agree with Ono that shred is a good tool for this. If you don't want to use that, try increasing the block size to at least 1M if not 16M to reduce the overhead.