How do I copy files from macbook to external harddrive using single user mode?
On newer OS it's far better to boot to Recovery HD or Internet Recovery and then open terminal to validate that things are mounted and running properly.

From there, you can use Disk Utility and / or Terminal to copy files off a non-bootable OS.


https://support.apple.com/en-us/HT201314


The benefit here is the system boots more fully on new Mac, avoids needing to know if you have T2 chip / FileVault and also mounts the attached drives when you start Terminal from the utilities menu.

Old school single user mode is still valid for non-T2 machines and if you can get past FileVault full disk encryption with a known admin / unlock account and password.