You can use the dd and nc commands for exact disk mirroring from one server to another. The following commands send data from Server1 to Server2:

Server2# nc -l 12345 | dd of=/dev/sdb

Server1# dd if=/dev/sda | nc server2 12345

Make sure that you issue Server2's command first so that it's listening on port 12345 when Server1 starts sending its data.

Unless you're sure that the disk is not being modified, it's better to boot Server1 from a RescueCD or LiveCD to do the copy.