Based on other comments here, typically the USB key is destroyed after the data was copied into the network. No data is allowed to exit the airgapped network.

Based on other comments here, typically the USB key is destroyed after the data was copied into the network. No data is allowed to exit the airgapped network. Read-only media or destroying the media after use is a reasonable mechanism to protect against data exfiltration. I'm not sure how you protect against infiltration though. A computer system that cannot get data in is pretty useless methinks.