My employer (Fox-IT) sells a data diode (https://www.fox-it.com/en/products/datadiode/ https://www.fox-it.com/en/products/datadiode/). List price is "call us fo

My employer (Fox-IT) sells a data diode (https://www.fox-it.com/en/products/datadiode/ https://www.fox-it.com/en/products/datadiode/). List price is "call us for a quote", but the quote won't be anywhere near $1 MM. Using a "proper" diode instead of hacking something yourself gets you a guaranteed-good solution - how much do you trust your firmware? - plus some software that automates "I want to send X through this machine" for many common and/or high-value instances of X. That said, custom hardware plus custom software plus certifications plus enterprise sales is indeed (a lot) more expensive than snipping the tx wire/fiber. Automatically copying USB sticks doesn't seem particularly useful to me.