I needed to create a copy of a Mac mini (which runs usually Windows 10, but also has a macOS partition on it) to a spare machine, so that I could easily replace one by the other in case of an emergency.

I decided to use a block-level copy of the SSD from my source Mac to the target Mac, by using dd from the Terminal available when running the Mac in Recovery Mode.

Terminal work from macOS in Recovery Mode

By getting down to Recovery Mode I can unmount the system disk and create a copy of it, without having to worry about the OS using it, or restricting my access…

Prepare the Target Mac

• Press Alt and power on the target Mac mini.
• When the boot menu is displayed, press Cmd-T.
• The Mac displays the Target Mode logo (lightning bolt).
• Attach the target Mac to the source Mac using a thunderbolt cable.

Prepare the Source Mac

• Press Alt and power on the source Mac mini.
• When the boot menu is displayed, press Cmd-R.
• After macOS booted into Recovery Mode, use the Tools menu and open a Terminal.

Copy the disk

This is the scary part. Make sure you don’t accidentally swap the source and the target of the copy!

• At the command prompt, execute diskutil list.
• In the list, identify the internal, physical disk (usually /dev/disk0).
• In the list, identify the external disk (in my case, this was dev/disk4).
• Use dd if=/dev/rdisk0 of=/dev/rdisk4 bs=1m conv=noerror,sync to copy from disk 0 to disk 4. Note the use of rdisk (raw disk) instead of disk when used with dd (you’ll get better performance with the raw disk device).

In case dd reports resource busy on one of the disks, you’ll have to unmount it, for instance with diskutil unmountDisk /dev/disk4.

How does is do?

dd is not very chatty. After pressing Return the command executes without any visual feed-back. In my setup, with two Mac mini (Model No. A1347 with 1TB SSD and 16GB RAM), I get a throughput of roughly 130MB/s. Copying 1TB block by block requires more than 2 hours.

The main thing: it gets the job done without any additional tool.