I think what you are asking about is an "air gap".
Here's a blog entry from Bruce Schneier with some best practices:
On Wed, Aug 23, 2017 at 8:44 AM, Coates, Sarah <[log in to unmask]> wrote:
> Asking on behalf of our library's Systems department: Does anyone have any best practices for a non-networked/quarantined computer for confidential information/sensitive information? We are looking for ways to get more potential donors of digital material, but some of these donors do not want their work to be on a networked computer.
> Our Systems department is looking for a methodology or best practices for a computer that can be backed up/connected to an internal-only network (with no public access) but that cannot access networked resources and the internet. If your archives has something like this set up, would you be willing to share your set-up?
> Please feel free to respond off-list to Scott Lewis at [log in to unmask] Scott can also answer any questions if you have any.
> Thank you so much in advance!
> Sarah Coates, MA, MLIS, CA
> 204 Edmon Low Library
> Oklahoma State University
> Stillwater, OK 74078
> [log in to unmask]