Our tiny two-person organization has a requirement to transfer sensitive documents (in both directions) between an air gapped computer and an Internet connected computer (assumed compromised). The air gapped computer stores a large volume of sensitive documents whilst the online untrusted computer stores very low volume (accepted risk). We need to move documents once per week for archiving to the air gapped computer and in some rare cases have a requirement to retrieve sensitive documents to the online untrusted computer (to forward to 3rd parties).The threat model excludes TLA's but includes targeted attacks from sophisticated cyber-criminals. Our primary concern is unauthorised access to the air gapped computer and exfiltration of sensitive documents through malware or other attack vector.My solution for retrieving documents from the air gapped PC is to use a USB firewall device (e.g.  enforcing write protection when connected to the untrusted PC.However we also need to move files (for archiving) from the untrusted PC to the air gapped PC. So far we have considered serial port but its too slow for data volume required (150-500MB per week). We have also considered burning CD's - This would mitigate against USB firmware related attacks but its not clear that malware can't be transferred when mounted on trusted computer. A data diode would seem a good solution but are cost prohibitive. Are there any solutions that could handle 150-500MB per week that we haven't considered?Thank you.