I bought external drive and the Apple technician working to backup my stuff at store using the method you said.
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It can be done from another Mac using Target Disk Mode:
If you don't have access to another Mac and you are bringing it in yourself, they may be able to do it in the store.
backing up from the command line via single user mode.
Question: My MacBook Pro is going in for service. Online help couldn’t get me to desktop. I’m got message saying to backup my laptop before I go.