Refer to the method here: MacBook Pro system failure, can not start, how to copy the data in the hard disk?
My approach:
First, mount the Macintosh HD disk through Disk Utility (the system will not automatically mount it at first), and then through the terminal, use the nc command to transfer the data to the unix-like system (because nc is also used to build the server side). The reason why it is not directly copied to the mobile hard disk in the terminal is because the mobile hard disk is mounted in read-only mode on the mac.