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I own a MacBook Pro purchased in 2014 that was shipped with OS X Mavericks already installed; since then, I have never upgraded to newer releases of the system software because I didn’t like Apple’s growing policy of restricting their customers in the use of their computers, but now I find myself almost forced to give up my obstinacy and to finally upgrade to the most recent version of macOS. Before doing so, however, I’d like to set up a way to roll back to Mavericks should I ever want to. I read this answer to Rolling back to a previous version of OS X Mavericks, which suggests “to attach a USB HD and clone the [startup] drive”. But how can I do this? How do I clone the startup disk?

Edit: Solutions that require the use of command-line tools are perfectly acceptable.


Addition

I succeeded in cloning the disk with the dd command; but when I try to set the cloned disk as the startup disk, by means of the relevant pane in the System Preferences application, an error sheet appears, with the following message:

You can’t change the startup disk to the selected disk.

The bless tool was unable to set the current boot disk.

Note that the cloned disk shows up in the wiew that presents the available startup disks to choose from.

If I try to set the startup volume from the command line, saying (after obtaning root priviledges)

bless --mount /Volumes/<volume name> --setBoot

the command executes without complaining, but its exit status is 3. And indeed, if I subsequently ask

bless --info --getBoot

the answer confirms that the boot volume has not been changed.

What am I doing wrong?

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