Boot both machines via usb†, start ssh server on one Depending on where the ssh server is (source or target): dd if=/dev/sda | ssh -C 1.2.3.4 dd of=/dev/s

Boot both machines via usb†, start ssh server on one Depending on where the ssh server is (source or target): dd if=/dev/sda | ssh -C 1.2.3.4 dd of=/dev/sda ssh -C 1.2.3.4 dd if=/dev/sda | dd of=/dev/sda † or mount the source / as ro, or systemd-switchroot to a ramdisk. Just make sure that source / is not getting writes. Or not, if you don't care.