The macOS System resides on a "Signed System Volume". This volume is cryptographically sealed, and that seal can only be applied by Apple; ordinary copies of the System volume are non-bootable without Apple's seal. When you configure a CCC backup task using the Legacy Bootable Copy Assistant, CCC will automatically use Apple's proprietary APFS replication utility (ASR) to make a block-for-block exact copy of the source. If that does not produce a bootable volume, and if you have exhausted the Firmware Discoverability Troubleshooting steps below, then we recommend that you install macOS onto the backup. If that does not produce a bootable device, then the device is not suitable for functioning as a bootable device on your Mac.
Only Apple can make an external device bootable. Our support for system copying and bootability is limited to the suggestions noted above.
- Troubleshooting APFS Replication
- Creating legacy bootable copies of macOS
- Installing macOS onto a CCC backup
No Mac will ever boot from an OS that is older than what it shipped with
Apple has never supported booting a new Mac from an OS that is older than what it shipped with. If you're trying to migrate content to a new Mac, use Migration Assistant for that purpose — do not attempt to restore an older Mac's backup onto a new Mac.
- Can I restore my Mac's backup to another computer?
- Apple Kbase #HT204350: Move your content to a new Mac
When you boot your Mac while holding down the Option key (Intel Macs) or the Power button (Apple Silicon Macs), the Mac Startup Manager will display a list of available startup devices. Using only device drivers that are stored on your Mac's firmware chip, the firmware will scan all of your SATA, PCI, USB, and Thunderbolt busses for hard drive devices, then read those hard drive volume headers to determine if a macOS system is available on each volume. Ordinarily, a CCC-created copy of your system volume will appear in this list, but occasionally your Mac's firmware may have difficulty discovering the hardware that hosts your backup.
Here are a few common reasons for devices to not appear, and then some troubleshooting steps you can try to resolve it.
If you are attempting to boot a Mac with an Apple T2 controller chip (e.g. a 2018 MacBook Pro or an iMac Pro) from your CCC bootable copy, be sure to change your Mac's External Boot policy to allow booting from an external hard drive. Apple describes the procedure in this Apple Kbase article, but the steps are:
- Restart your Mac while holding down Command(⌘) and the "R" keys.
- Choose Startup Security Utility from the Utilities menu in the menu bar (see this screenshot for clarification)
- Click the Enter macOS Password button, then choose an administrator account and enter its password.
- Change the External Boot (or "Allowed Boot Media") setting to Allow booting from external media
Please do not, however, change the Secure Boot setting for the purpose of booting from a backup. "Full Security" is the default setting, and that setting is compatible with booting a T2 from its own backup. Do note the exception to this when attempting to boot one of these Macs from a different Mac's backup – booting a T2 Mac from another Mac's backup is not supportable.
Note for users with non-QWERTY keyboards: When you initially boot into Recovery mode, you'll be prompted to select a language. Be sure to select a language that matches your keyboard, otherwise the Startup Security Utility may not accept your password.
Can I leave this setting unchanged and change it only in the future when I actually need to boot from my backup?
Generally no. Changing settings in the Startup Security Utility requires a functional user account on the internal disk of your Mac. If your Mac's startup disk were to fail, it would be impossible to change the startup security settings. Because the primary purpose of a bootable backup is to function as a rescue disk in the event that your Mac's startup disk fails or otherwise becomes non-functional, we recommend leaving your Mac configured to allow booting from external devices.
For additional startup security, you can apply a firmware password. When a firmware password is applied, your Mac will require a password to load the Startup Manager on startup.
Rule out generally incompatible configurations and filesystem anomalies
If you are using an external hard drive enclosure, see whether your enclosure is listed at the bottom of this page as an enclosure that we've seen problems with in the past.
Eliminate all hubs and adapters between your Mac and the external storage. Your external device should be plugged directly into a USB or Thunderbolt port on your Mac using a cable that has the correct plugs on each end (no adapters).
- Turn off your Mac
- Detach all peripherals from your Mac except for the keyboard and mouse (including any secondary displays)
- Attach the backup disk directly to a USB or Thunderbolt port on your Mac (no hubs, no adapters, no monitor ports, no daisy chaining, no third-party USB cards)
- Start up your Mac while holding down the Option key (Intel Macs) or the Power button (Apple Silicon Macs). [Note: A wired keyboard may be required for this step]
- Wait about 30 seconds to see if the backup volume appears. If your backup volume appears at this step and the boot process proceeds past the Apple logo, skip to the section below.
- Press Option-Shift-Command-Period at the Startup Manager window to load any Option ROM firmware that is present and required for an external hard drive enclosure.
- Detach, then reattach the backup volume's USB or Thunderbolt cable from/to your Mac and wait up to another 30 seconds. If your backup volume appears, select it and proceed with the startup process.
- If the backup volume still does not appear as an option, shut down your Mac completely. Then start it up holding down the Option key (Intel Macs) or the Power button (Apple Silicon Macs), waiting another 30 seconds for the volume to appear.
- Repeat the steps above, but using another interface (e.g. USB if you tried Thunderbolt, Thunderbolt if you already tried USB) and see if the volume appears.
- If the hard drive enclosure is bus powered, try plugging in its DC power supply before starting up your Mac. Bus powered enclosures often take a bit longer to spin up or simply don't make themselves available that early in the boot process.
Here are a couple additional steps you can perform to try to get your Mac to "see" your USB device early in the startup process.
- Reboot your Mac while holding down the Option key (Intel Macs) or the Power button (Apple Silicon Macs).
- If your Mac has multiple USB ports, try attaching your destination disk to each port (and be sure to use the ports on your Mac directly — not a hub, keyboard, or display)
Reset the Mac's Parameter RAM
Lastly, try resetting your Mac's parameter RAM. PRAM maintains settings related to starting up your Mac, and it's possible that invalid settings are interfering with your Mac's discovery of the external enclosure. To reset your PRAM on an Intel Mac:
- Hold down Command+Option+P+R on startup
- Hold down those keys until you hear the second startup chime.
- Release all but the Option key after you hear the second startup chime.
If the volume still won't boot, it may be impossible for your firmware to detect your enclosure (despite that macOS, once booted and having access to far more device drivers, can see the enclosure just fine). The Golden Litmus Test for bootability would be to install macOS directly onto the volume. If the macOS Installer fails to make the disk bootable, then your external device will not function as a startup device.
The backup volume starts to boot the Mac, but fails to get to the Finder, or the Mac reboots and boots from the internal disk
If your backup volume showed up in the Startup Manager, and you selected it and proceeded with the startup process, but...
Your Mac doesn't display the Apple logo (e.g. you get a blank, black or gray screen after selecting the backup volume): try installing macOS onto the volume to verify its suitability as a startup device.
The Apple logo and a progress indicator appears, but the startup process never completes (and perhaps the Mac reboots from the internal disk): There may be an storage driver conflict at play, or a compatibility issue specific to the enclosure.
- Choose "About This Mac" from the Apple menu to verify that your Mac really did not boot from the volume that you selected
- Detach all unnecessary peripherals, including secondary displays.
- Reboot the Mac and hold down Option (Intel Macs) or the Power button (Apple Silicon Macs) to load the Startup Manager
- Select the backup disk
- As you click the button to proceed with the startup process, hold down the Shift key to boot in Safe Boot mode
If your Mac successfully boots from the selected volume in Safe Boot mode, open the Terminal application and paste in the following command:
sudo kextcache --clear-staging
sudo kextcache -system-prelinked-kernel
sudo kextcache -system-caches
Press the Return key after pasting in each line and authenticate when prompted. Then try again to boot from the same volume without Safe Boot mode. If your Mac still fails to boot from the selected volume, try installing macOS onto the volume to verify its suitability as a startup device.
The performance of your Mac while booted from the backup depends almost entirely on the performance of the hardware, and more specifically, the performance of the filesystem on that hardware. If your backup disk is an SSD, you can expect very good performance — comparable to the performance that you get when you boot your Mac from its internal SSD. If your backup disk is a rotational HDD, then performance will vary from adequate to very poor, depending on the specific performance characteristics of your backup disk. In particular, Apple's APFS filesystem performs relatively poorly on rotational HDD devices, and that performance is considerably worse for 5400RPM disks and disks that use Shingled Magnetic Recording. You may find the performance of one of these slower HDDs to be unusable for booting your Mac.
- Choosing a backup drive: Devices that we recommend
- Using Migration Assistant to restore your startup disk from a CCC backup
- USB thumb drives, flash drives and SD cards are inherently slow devices, we don't recommend using these for making a bootable copy of the system.
- Flash-based memory like that used in SD cards and thumb drives also has limited write/erase cycles that are much lower than that of a traditional SSD or mechanical hard drive making them inappropriate as a primary backup device.
- We have received a report that the Nexstar 6G USB 3.0 hard drive enclosure may not bootable due to a discoverability issue.
- We have received several reports that multiple-bay hard drive enclosures provide inconsistent boot results. In each case, the Mac can boot from the external disk as long as the hard drive is placed in the first bay of the enclosure. When placed in other bays, the bootable volume is not discoverable by the Mac's firmware. If you have trouble booting from a disk in a multi-bay enclosure, try swapping the drive positions within the enclosure.
- We have received a report that the Orico 3588US3 USB3 enclosure is not bootable due to a discoverability issue.
- Some users have reported discoverability issues with ASM1352R enclosures from ASMedia.
- One user reported that the MyDigitalSSD Boost enclosure is not bootable.
- Sonnet Customer Support has confirmed that any device attached to the Sonnet Allegro Pro USB 3 PCI card cannot function as a startup disk.
- Some users have reported bootability issues with the Inateck USB 3.0 2.5" hard drive enclosure with a model number of "FEU3NS-1".
- We have received a report that the Sabrent Rocket Pro 2TB NVMe USB 3.1 External Aluminum SSD is not bootable.
- We have received at least two reports that the LaCie d2 is not bootable.
- We have received a report that the GMM M.2 NVMe enclosure is not bootable (test case was a MacPro running Monterey, confirmed by booting the same storage in another enclosure).
- We have received a report that the the Samsung SSD drivers (i.e. those provided by Samsung) cause macOS to either stall or kernel panic when attempting to boot from a Samsung T7 SSD. We recommend that you avoid installing the Samsung storage drivers, they are redundant to the built-in macOS storage drivers.
- We have received several reports that various external devices fail to boot macOS. The common thread in these reports is that the macOS Installer hangs with "one minute remaining", and never completes the installation procedure (and users are trying the installer as a last resort). As noted above, if the macOS Installer can't produce a bootable installation of macOS on your external device, then that device is simply not going to be capable of booting your Mac. We recommend using that storage for a Standard Backup instead.