This is a bit of a hypothetical, but I am interested in an answer. First, I understand that no one can protect themselves against three letter organizations, so I am really just wondering about best practices.
Let's say I have a computer that is not connected to the internet. The computer doesn't even have a modem or wireless capabilities. The computer is used for working on secrets. You might be using the computer to run computations on certain data. Let's say that this data comes from a computer that is connected to the internet. So, one needs to, say, daily, transfer data from the internet connected computer to the secret computer.
The data itself isn't secret, but the outcome of the work done on the secret computer is. No data ever needs to electronically leave the secret computer.
My question is: What is a best practice for transferring files between the two computer?
One problem is that is the same USB key is used every time, then this USB key might be infected and indirectly allow an adversary access to the secret computer. One solution I can think of is to use a new USB key for every transfer, so that a USB key is destroyed after it has been connected to the secret computer. That way no secret work can make it's way to the internet connected computer.
But: Is there a way that doesn't require a new USB key every time?