Boot from external drive

I have a May 2015 MacBook Pro Retina 2.5GHz 15.4" currently running macOS 12.4 “Monterey”.
Having for many years had external backup drives for a series of Macs and being able to boot from those drives, I bought an Envoy Pro Elektron 2TB SSD drive from Other World Computing. Upon trying to copy the internal drive to the new external drive using Carbon Copy Cloner, I learn that it is a difficult and dangerous (?) thing to do.
I would hate to think that if the internal SSD fails the MacBook Pro will become a door stop.
How do other people cope with Apple’s new and, to me, weird operating system requirements?
For now I’m doing all my work with a 2017 iMac 27" running macOS 10.14.6 Mojave. I only have the MacBook Pro upgraded to the latest OS to run TurboTax.Yes, I assume not updating the iMac to the latest OS is a security risk.

On my 2017 iMac 27", the original internal Fusion Drive was really slowing down the system, so rather than replace it I got an external OWC Envoy Pro FX 2T SSD, installed (at the time) Big Sur cleanly, then booted from the SSD and used Migration Assistant to complete the move from to the external drive. I’m now running the SSD as my boot drive running Monterey 12.5, and the Fusion drive has given up the ghost with SMART errors. Best thing I ever did, the money on the SSD was spent wisely.

So I recommend that you do a clean install of Monterey on the SSD and then use Migration Assistant to bring everything over to the SSD, and use it as your boot drive.


The MacBook Pro probably is APFS formatted, CCC can not clone the system partition (container). Use the Monterey installer to install the system onto your SSD and use Migration Assistant to copy your data.


And after you’ve used the Apple installer to install Monterey, CCC will have no problem smart updating your data partition per your set schedule. That’s what I’m doing. The key is you have to use the Apple Monterey installer to first install the system on each external partition you want to update with CCC. Then they’ll be bootable. (Well, as long as they’re not RAID…Monterey flatly refuses to boot from a RAID array.)


@terryk I would love to see how that job is set up in Carbon Copy Cloner. The recent changes to macOS and APFS have had me baffled for setting up bootable backups, and I’ve just settled on backing up only the data partition. Installing Monterey onto an external drive is straightforward. It’s setting up the subsequent CCC job that seems tricky to get right.

@terryk said, “I would love to see how that job is set up in Carbon Copy Cloner.”

Easy peasey, here is instructions for you.

  • Connect a backup drive with an APFS volume configured.
  • Launch CCC.
  • Create a new Backup Task and name it appropriately – click on the task name under Tasks to edit it.
  • Select Volumes.
  • Drag the ‘- Data’ volume to the SOURCE pane.
  • Drag the target volume to the DESTINATION pane.
  • Save the edited task.
    The result is a basic and correct CCC task. Run it in whatever manner is convenient.

As a minimum in my environment Preflight, I set SafetyNet to Off, run a script to stop Time Machine, and uncheck Protect root-level items. In Postflight I enable Time Machine, dismount the destination volume, and send email. Depending on specific machines, I may chain to backups of additional userData volumes. Periodically, as before major upgrades, I check re-verify the files that were copied. The exact configuration is driven by business needs.

Backing up the ‘- Data’ volume is exactly correct for Monterey and follow-on macOS APFS systems. The ‘- Data’ volume contains user-installed applications as well as user files. As has been mentioned previously, macOS should be installed it a dedicated partition – either as an initial install or a repair/upgrade install. Use of CCC to clone the ‘- Data’ volume to the install target guarantees, as much as is possible, an identical copy of the original volume.

Any new or upgrade installation of macOS to a drive, either internal or external, is independent of ‘- Data’ volume. Not since the 1990s VAX/VMS systems has OS upgrading/repairing been easier.

Also, once macOS has been installed, booted, and updated on any drive, Migration Assistant can source a CCC backup, handling all the firmlink and other complexities required for a proper APFS macOS system.


As James Cutler has given you a good explanation of how to start, I’ll just add if you want to review CCC instructions on the web, they’re available here: CCC - How to set up your first backup.

Once you’ve installed Monterey onto the external drive, just select that same drive in CCC as the destination and it’ll automatically select the “-Data” partitions of each drive for you as both the source and destination of that task.

One caveat I’ve found: I’ve only been successful booting a Thunderbolt (2, 3, or 4) connected external drive. USB 3 drives throw up an error upon attempting to boot after a CCC clone task has been performed. The error indicated the EFI partition was damaged, which I think may be related to the “Updating boot partition” dialogue one sees as the final step of the cloning process.

I tested this by using Migration Assistant to clone the Data partition initially, which booted without issue. But immediately after CCC cloning to the same partition, the error occurs. I repeated this many times with the same result. (This was when I had an Intel Mac; I don’t know if this same error occurs with Apple Silicon or not.) I don’t have an answer for that. I just ditched my USB 3 drives and bought Thunderbolt drives instead.