An endless supply of 1's for securely overwriting and erasing a disk
The file for endless ones is located in /usr/bin/yes 1 - you can often use the backtick character ` to execute that program in place, but you could also set up a named pipe if you really wanted a place to get ones.

Rather than use dd - you might have much better luck with a tool designed specifically for the task.


https://github.com/rentzsch/stressdrive - source code to verify 100% overwrite
diskutil zeroDisk disk3
diskutil secureErase 4 disk3


The level for secureErase can be 0-4 according to the man page and your taste for the number and types of overwrites.