Login to Hetzner's web interface https://robot.your-server.de/ and activate the Rescue System for the server whose disk you wish to clone.
Ssh into the Rescue System (Replace 1.2.3.4
in the command below with the actual ip address of the server)
ssh -o HostKeyAlias=hetzner-rescue.1.2.3.4 root@1.2.3.4