The easiest way is to use SSHand DD or a combination of netcat. SSHwill be a little slower due to encryption but is the most secure way (on two older systems the average clone speed is about 40-50MB/s). This is also OS independent as it doesn't matter what the source OS is because you are literallly cloning the drive so you retain the partition table and settings.
Clone HDD using SSH and DD........
The easiest way is if you have a secure way to connect and verify the hostkey of the remote host by using this guide.
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@ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @
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IT IS POSSIBLE THAT SOMEONE IS DOING SOME........