Carbon Copy Cloner
Knowledge Base > Carbon Copy Cloner 6

External Boot Troubleshooting

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This documentation is for an older version of CCC. You can find the latest version here.
Last updated on October 18, 2023

We're happy to help you troubleshoot bootability problems affecting macOS Catalina. Only Apple can make an external device bootable with macOS Big Sur, however, so our support for external boot on Big Sur and later OSes is limited to the suggestions offered below.

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.

Related Documentation

External Boot troubleshooting on macOS 11, "Big Sur" and later OSes

Starting in macOS Big Sur, the system now 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.

Our support for system copying and bootability on Big Sur and later OSes is limited to the suggestions noted above.

Related Documentation

Sometimes the Mac's firmware cannot detect your backup device

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 bootable backup volume will appear in this list, but occasionally your Mac's firmware may have difficulty discovering the hardware that hosts your backup.

If CCC's Task Plan didn't report any configuration concerns for your backup volume and you are having trouble booting from it, try the Firmware Discoverability Troubleshooting steps below.

Some Macs may not boot from USB devices larger than 2TB

Some Macs, especially those produced prior to 2014, cannot "see" the content of a volume that lies past the 2TB mark on the disk at boot. If you have an older Mac and you're having trouble booting it from a USB device that is larger than 2TB, try creating a 2TB partition at the beginning of the disk and make your backup to that partition. Note that when partitioning a disk in Disk Utility, the top of the pie chart is the beginning of the disk; in other words, the first partition starts at "noon".

Possible workaround: If your external device has a Firewire interface, and your Mac is running an OS that is older than Catalina, then you can attach the device to your Mac via Firewire and boot from any size of volume. If your Mac does not have a Firewire port, but has Thunderbolt ports, you can use the Apple Thunderbolt to Firewire adapter.

2012-vintage Macs can't boot macOS Catalina from an encrypted USB device

We have received several reports that the 2012 Mac mini and the 2012 MacBook Pro can initially boot from a non-encrypted external USB device, but then will fail to boot from that device when FileVault is enabled on the external device. This issue is not specific to CCC, we have confirmation that this occurs when installing Catalina directly onto an external device as well. This problem does not appear to be specific to any particular enclosure, rather it appears to be specific to the 2012 models of Mac mini and MacBook Pro. If you require an encrypted backup, we recommend that you erase your destination as APFS or HFS+ encrypted, then create a data-only backup to that volume.

We reported this issue to Apple (FB7433465) in November 2019 and we are currently awaiting a response.

Apple no longer supports booting Macs from RAID devices

Starting in macOS Mojave, Apple no longer supports installing macOS onto a RAID device. Some people have found that backing up Mojave to a RAID array can work, however this is not a supported configuration, and does not appear to be a viable option for macOS Catalina.

Enable "External Boot" on T2 Macs (this is not required on M1 Macs)

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 backup, 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:

  1. Restart your Mac while holding down Command(⌘) and the "R" keys.
  2. Choose Startup Security Utility from the Utilities menu in the menu bar (see this screenshot for clarification)
  3. Click the Enter macOS Password button, then choose an administrator account and enter its password.
  4. Change the External Boot (or "Allowed Boot Media") setting to Allow booting from external media
  5. Restart

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.

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.

Apple Kbase HT204455: How to set a firmware password on your Mac

Make the Startup Manager load additional drivers

Some third-party external devices use Option ROM firmware. Macs with "up-to-date software" don’t automatically load Option ROM firmware, so your Mac won't see devices that have Option ROM firmware until you load that firmware. Press Option-Shift-Command-Period at the Startup Manager window to load Option ROM firmware from any currently-attached hard drive enclosures. Here's a partial list of devices we've received reports of that use Option ROM firmware:

Rule out generally incompatible configurations and filesystem anomalies

If you are using an external hard drive enclosure or adapter, see whether your enclosure is listed at the bottom of this page as an enclosure that we've seen problems with in the past. Also, for good measure, use Disk Utility's "First Aid" utility to verify and repair any filesystem problems that may be present on the destination volume.

Troubleshoot discoverability issues in the Mac's Startup Manager

  1. Turn off your Mac
  2. Detach all peripherals from your Mac except for the keyboard and mouse (including any secondary displays)
  3. 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)
  4. 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]
  5. 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.
  6. 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.
  7. 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.
  8. 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.
  9. 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.
  10. 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.

Additional USB device troubleshooting

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.

  1. Reboot your Mac while holding down the Option key (Intel Macs) or the Power button (Apple Silicon Macs).
  2. 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)
  3. If you are using a USB 3.0 enclosure, try using a USB 2.0 cable (yes, it will work!). USB 3.0 devices are backwards compatible to USB 2.0, but they don't always play well with the older USB device drivers that are embedded within your Mac's firmware. Using a USB 2.0 cable elicits different behavior from the enclosure that often works around compatibility problems that are only exposed when using the Mac's firmware USB drivers. Here are some pictures that show what the ends of USB 2.0 and USB 3.0 cables look like:

USB 2.0 Micro vs. USB 3.0 Micro

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:

  1. Hold down Command+Option+P+R on startup
  2. Hold down those keys until you hear the second startup chime.
  3. Release all but the Option key after you hear the second startup chime.

Definitively rule out an incompatible enclosure

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): your Mac is having trouble finding the "booter" file on this volume. This can occur due to hard drive enclosure interference, due to filesystem corruption on the backup volume, or due to the volume being improperly "blessed" (blessing a volume stores certain information about the startup files in the volume's header, and your Mac uses that information to start the boot process).

  1. Erase the backup disk, then back up your startup disk to the destination again.
  2. Try booting from the backup volume again.

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 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 extension conflict at play, or a compatibility issue specific to the enclosure.

  1. Choose "About This Mac" from the Apple menu to verify that your Mac really did not boot from the volume that you selected
  2. Detach all unnecessary peripherals, including secondary displays.
  3. Reboot the Mac and hold down Option (Intel Macs) or the Power button (Apple Silicon Macs) to load the Startup Manager
  4. Select the backup disk
  5. 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, 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.

Related documentation:

Performance expectations while the Mac is booted from the backup

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 format of the backup volume, the operating system version, and 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 the purpose of booting your Mac from the backup.

Related documentation

If you see the universal "No access" symbol after selecting your startup disk

This indicates that the macOS cannot load the the startup files, or that it cannot mount the startup disk:

No Access Symbol

The most frequent cause for this is an attempt to boot your Mac from an incompatible (i.e. too old) operating system. Occasionally this also occurs due to a device driver conflict with the enclosure you are trying to boot from, or due to a firmware compatibility problem between the Mac and the enclosure. We occasionally see this when trying to boot pre-2013 Macs from a USB 3.0 enclosure. We also see this more frequently on Yosemite when a critical kernel extension's code signature is invalid. This can happen, for example, when using something like TRIM Enabler to modify macOS Storage drivers.

  • Apple does not support booting a Mac via a FireWire-attached device. If your device is attached via FireWire and has a USB port as well, try attaching the device to your Mac via USB.
  • Try booting into Safe Boot mode (hold down the Option key (Intel Macs) or the Power button (Apple Silicon Macs) on startup, then hold down the Shift key as you select the backup volume as the startup disk).
  • Try installing macOS directly onto the backup volume while your Mac is booted in Recovery mode. If the installation also fails, there is a compatibility issue between the enclosure and your Mac that makes it unsuitable as a startup device.
  • If you used a third-party utility to modify macOS software (e.g. TRIM Enabler), undo that modification, then run the backup task again.

If your Mac never progresses past the progress indicator (below the Apple logo) or stalls at the Apple logo+progress bar while booting from the backup volume, there is probably a problem with some of the system files that are called early in the startup process, or macOS is unable to load the correct drivers for your external enclosure at that stage of the startup process. Again, try installing macOS directly onto the backup volume while booted in Recovery mode to rule out a compatibility problem with the enclosure.

"unapproved caller. security agent may only be invoked by Apple software" message appears on startup

This message generally appears when the volume you are trying to boot from is full or nearly full. You can remove items from the _CCC SafetyNet folder (or the entire folder itself), then empty the Trash, or remove snapshots from that volume to free up some space before trying to boot from that volume again. macOS should be given at least 2GB, preferably 5-10GB of free space to accommodate the creation of cache and virtual memory files on startup.

Related documentation:

The Mac boots from the backup, but login fails

We have received a handful of reports that login is denied despite providing the correct password, and despite that the user accounts database and password storage is copied correctly to the backup volume. You can correct the problem while booted from your production startup volume:

  1. Open the Users & Groups Preference Pane in the System Preferences application
  2. Click the "Change Password" button
  3. Re-enter your current password (in all three fields – reusing the current password is fine) and add a hint as well. The hint doesn't have to be anything meaningful, just something you can verify later, like "meatball".
  4. Run the backup task again
  5. Try again to boot from the backup disk and log in

"You can't change the startup disk to the selected disk. The bless tool was unable to set the current boot disk."

Occasionally the Startup Disk Preference Pane will issue this error without any useful context. More often than not, the inability of the Startup Disk Preference Pane to change the startup disk is not actually an indication that the volume will not be bootable, rather it simply means that the Startup Disk Preference Pane cannot change the startup disk selection to that particular volume. Use the Startup Manager (boot your Mac while holding down the Option key [Intel Macs] or the Power key [Apple Silicon Macs]) to select an alternate boot disk instead.

System Integrity Protection prohibits modifications to the current startup disk's Preboot helper partition

If you add an APFS volume to your current startup disk's APFS container, the macOS bless facility will be unable to update the container's Preboot volume to include support files for the second partition. System Integrity Protection will also prevent the preservation of system files on any other volume that resides in the same APFS container as the current startup disk. As such, CCC will exclude system files when you configure a task with a destination that is in the APFS container of the current startup disk.

Alternatively, you can create a separate partition on your startup disk (rather than adding a second volume to the same parent APFS container) and make your backup to that separate partition.

  1. Open Disk Utility
  2. Choose "Show all devices" from the View menu
  3. Click on the top-most parent device for your Macintosh HD volume
  4. Click the "Partition" button in the toolbar
  5. When Disk Utility tries to discourage you from doing this, by preselecting "Add Volume" click the "Partition" button
  6. Click the "+" button to add a second APFS-formatted partition on the startup disk

Configurations with which we have seen some problems

  • USB thumb drives are inherently slow devices, we don't recommend using these for making a bootable backup.
  • We have received many reports of inconsistent bootability with SanDisk flash drives (Cruzer, Ultra) and SD cards on macOS High Sierra. These devices are often slow anyway, so we don't recommend using these specifically for a bootable backup. Catalina+: The same issue that causes bootability problems with these devices on pre-Catalina OSes now causes errors that prevent even a basic backup of the System and Data volumes. We recommend using these devices only for creating a non-bootable backup of your Catalina Data volume.
  • 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 not appropriate as a primary backup device.  
  • Western Digital enclosures have an unreliable track record at serving as startup devices. Incompatibilities include:
    • A user reported that the WD My Passport Studio 2TB cannot boot a T2 MacBook Pro (this report was confirmed by an unsuccessful attempt to install macOS Mojave onto the device).
    • We have received many reports that Western Digital My Passport hard drive enclosures fail to function as a startup disk with macOS Catalina (again, confirmed by a failure to install macOS onto the device or to boot from that device after installing macOS via the Installer).
    • We have received a report that the Western Digital EasyStore fails to function as a startup disk with macOS Catalina (same confirmation as above).
    • Exception: The only Western Digital device that we have seen reliable results with is the WD MyPassport SSD.
  • Some Macs have trouble booting from USB 3.0 enclosures
  • We have received a report that the Nexstar 6G USB 3.0 hard drive enclosure is not bootable due to a discoverability issue. The Nexstar TX from Vantec was bootable (using the same internal hard drive). We have received another report, however, that the Nexstar 6G was bootable, so there may be Mac-specific firmware issues at play regarding this enclosure.
  • We have received several reports that multiple-bay hard drive enclosures provide inconsistent boot results. In each case, the Mac can boot from the bootable backup 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. Here is a list of the affected enclosures that we have had reports on so far:
  • We have received a report that the Orico 3588US3 USB3 enclosure is not bootable due to a discoverability issue.
  • We have received a report that agreeing to Webroot SecureAnywhere's request to "remove threats" during a backup task can produce a non-bootable backup.
  • Some users report problems booting pre-2013 Macs from USB 3.0 devices that use the "ASMedia 1051E" chipset (e.g. this OWC Mercury On-The-Go enclosure). A firmware compatibility issue was introduced by a 2015 firmware update to these Macs that prevents them from booting from a USB 3 device with that older chipset.
  • Some users have reported discoverability issues with ASM1352R enclosures from ASMedia.
  • One user reported that the MyDigitalSSD Boost enclosure is not bootable.
  • We have received a report that devices attached to the AmazonBasics 10 Port USB 3.0 Hub are not available in the Option-key Startup Manager. Attach your USB devices directly to a USB port on your Mac if/when you need to boot from your CCC bootable backup.
  • 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 a report that the 6-bay ThunderBay 6 from Other World Computing is not bootable on macOS Catalina. macOS proceeds ~75% of the way through the startup process, then stalls. The exact same disk placed into a different enclosure boots fine. In contrast, we have received a report that a non-RAID volume in a ThunderBay 8 enclosure from OWC was bootable on macOS Monterey. These are complex devices, so "your mileage may vary".
  • We have received at least two reports that the LaCie d2 is not bootable.
  • We have received a report that the VisionTek 1 GB Thunderbolt3 SSD is not bootable on macOS Big Sur (test case was a 2019 MacBook Pro, confirmed after the Big Sur Installer completed and the device failed to boot). In this particular case the device had been bootable on Catalina.
  • 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.

The 2019 iMac errantly boots from USB-C devices

We've been tracking an emerging issue specific to the 2019 iMac and external disks attached via USB-C (same port as Thunderbolt) in which the iMac will boot from the external device instead of the internal hard drive despite a preference to boot from the internal disk. We believe this is a problem in the firmware of this particular iMac — it's the firmware that decides which device to use as the startup disk, and it appears to be ignoring the user's preference (e.g. the internal startup disk). In one case a user performed a simple and definitive test — he installed macOS Catalina onto a freshly-erased, external device, and as long as that device was attached via USB-C, the Mac would only boot from that device, and regardless of the selected startup disk preference. This behavior is not specific to CCC nor to any particular enclosure, rather it seems to be a firmware bug.

Workaround: If your external hard drive enclosure came with a USB-C to USB Type A cable, then you could use that to connect the disk to a USB type A port on your iMac to avoid this issue. Or you could just detach the disk from your Mac prior to rebooting.

Related Documentation