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I have a 500 GB external USB disk that contains a 465GB "APFS container", with Mac OS X Catalina, bootable (with the trick "Press and hold the Option (Alt) key immediately after turning on or restarting your Mac")

I need to use this disk for something else, and I need to move it to another 320 GB disk.

How to do that?

  • First how to shrink the 465 GB to 300 GB (only < 100 GB are used)? Is it possible from another Mac OS High Sierra computer or another Linux or another Windows computer?

  • Then how to copy the partition to the 320GB so that it will still be bootable?

  • what computer and system are you running. The mac system has been over 200 MB for many years. I find it hard to believe the Mac system is on a 200 MB partition. – jmh Jun 17 at 18:40
  • @jmh this one is probably a boot partition? The OS and data are both on partition 2 - an APFS container – Basj Jun 17 at 19:15
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Apple documentation states you should be able to replicate a bootable APFS container to a new drive. Unfortunately, the GUI based software from Apple is wrought with bugs. Below are the steps I preformed to successfully clone Catalina macOS 10.15.5 to a new USB drive. To be honest, I admit this was tested using a VirtualBox virtual machine.

  1. Boot to macOS Recovery. I booted to the macOS Recovery APFS volume on the 465 GB USB drive.

  2. Plug the 320 GB drive in a USB port

  3. Open the Disk Utility.

  4. Under the View pulldown, select Show all Devices.

  5. Highlight the 320 GB Drive, then select the Erase button.

  6. Use the selections shown below, then select the Erase button.

  7. Quit the Disk Utility, then open the Disk Utility.

  8. Highlight the MyNewUSB volume, then select the Restore button. From the Restore from pulldown list, select the existing external USB macOS volume, as shown below. When finished, select the Restore button.

    Note: This will replicate the macOS, macOS - Data, hidden Preboot and hidden Recovery volumes.

  9. When finished, quit the Disk Utility, then open the Disk Utility.

  10. Visually, check the results.

  11. Quit the Disk Utility.

  12. Shutdown the Mac.

  13. Unplug the 465 GB USB drive.

  14. Start the Mac and immediately, hold down the option Key until the Mac Startup Manager icons appear.

  15. Hold down the control key and select to boot from the 320 GB USB drive. Note: This first boot took longer than normal.

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  • Thank you for your answer, I'll accept it, because it seems to be the solution! (In the meantime I finally reinstalled the whole system from scratch instead of cloning the disk... I had tried with SuperDuper without any success: the disk was non bootable at the end) – Basj Jun 18 at 12:53
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You can't really copy a partition, you just copy everthing thats in one partition to another partition of appropriate size. You should make a 320 GB partition on your new drive. Since you are only using 100 GB on the 465 GB partition you can copy everything in the 465 partition to the 320 partition. Just select everything and copy it using the Finder. To make the 320 GB drive bootable, you must install the system of your choice. You can istall the system in your 320 GB partition.

If your trying to copy to a different machine, after you copy everything to the 320 GB drive connect it to the other machine and move the data.

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  • Setting aside the other answer, there are at least two programs I can think of off the top of my head that can make bootable clones (CCC and SuperDuper!), so it's unclear, at best, what your answer is trying to say. Highlighting everything in Finder and copying it will not result in a bootable volume (which to be fair, you state), will definitely miss hidden folders, and you're going to get pieces of the old OS installation which would just confuse things further. Installing an OS over top of what you just copied is both nonsensical and will almost certainly lead to data loss. – tubedogg Jun 17 at 21:25

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