USB keys or SD cards are one way, but I dislike using USB drives because they are an unacceptable infection vector for my cold storage machine. USB, SD cards or any block storage introduces drivers and firmware which may be suspect and or may contain malware that you copy unknowingly or that gets launched automatically (reference: https://www.us-cert.gov/sites/default/files/publications/RisksOfPortableDevices.pdf)
What to do, then?
These laptops have a wired ethernet port. I can connect them back-to-back and simply have a static IP configured on both machines.
Enable the SSH server on the online machine with the following command:
sudo apt-get install -y openssh-server
Then go to the network menu, select Edit Connections, select Wired Connection 1 and hit Edit. Go to the IPv4 Settings tab and set Method to ‘Shared to other computers’. You can now hit Save and then Close.
Go back to the Network menu and select ‘Connection Information’ and note the IPv4 IP Address of Wired Connection 1. This is the IP address you will connect to from the cold storage machine.
Connect the two machines together with an ethernet cable.
Use FileZilla to connect to the IP address of your online host.
You can now safely drag and drop files between the two hosts.