We're happy to help you troubleshoot your bootability problems. Before you ask for help, please try the troubleshooting steps below. If you're having trouble with the steps or have run out of options, please let us know how far you got, or how far your Mac gets into the boot process.
Troubleshooting firmware discovery problems
When you boot your Mac while holding down the Option key, 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, Firewire, 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 following troubleshooting steps (in order).
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 configure your Mac to allow booting from an external hard drive. Apple describes the procedure in this Kbase article:
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 bootable backups.
Our testing has confirmed that Macs with Apple's T2 controller chip cannot boot from an encrypted, "Mac OS Extended"-formatted, external volume. Booting from an external volume works fine in general, but if your external disk is formatted using Apple's legacy HFS+, "Mac OS Extended" format, enabling FileVault on that volume will render it non-bootable, producing an error message like this on startup:
Spoiler alert: The "Update" option does not work. This may be a bug in the firmware of the T2 Macs, or it may be a limitation that Apple does not intend to address. In either case, if you want to encrypt your external, bootable backup of a T2-based Mac, we recommend formatting that backup volume as APFS.
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
- Turn off your Mac
- Detach all peripherals from your Mac except for the keyboard and mouse
- Attach the backup disk directly to a USB, Firewire, or Thunderbolt port on your Mac (no hubs, no daisy chaining)
- Start up your Mac while holding down the Option key. [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 next 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 Firewire, 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, waiting another 30 seconds for the volume to appear.
- Repeat the steps above, but using another interface (e.g. USB if you tried Firewire, Firewire 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.
Additional USB device troubleshooting
Macs' support for booting from USB volumes has always been second class to Apple-invented interfaces such as Firewire and Thunderbolt. We strongly recommend using Firewire or Thunderbolt for your bootable backup; those interfaces are consistently more reliable when it comes to bootability. If you have a USB-only device in hand, though, here are a couple additional steps you can perform to try to get your Mac to "see" it early in the startup process.
- Reboot your Mac while holding down the Option key.
- 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)
- 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:
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:
- 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.
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 that fails to make the disk bootable, then it definitely isn't going to happen. Please report these enclosures to us so we can assemble a list of troublesome enclosures.
There are several visual hints that can indicate how far your backup volume is getting in the startup process:
- Apple logo: The "booter" file was found and executed.
- Spinning progress indicator: The OS "kernel" was executed and now has control over the startup process. The kernel will load kernel extension caches, mount the startup disk, then execute "launchd" which kicks off all of the other system processes.
- Blue screen: The WindowServer has loaded, so the system is ready to start loading regular applications or the loginwindow.
- Loginwindow or your Desktop: The system has finished loading, and is ready for user interaction
If your backup volume showed up in the Option key startup disk selection screen, but doesn't display the Apple logo when you choose to start from it, then 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).
- Repartition the backup disk in Disk Utility, then reclone your startup disk to the destination.
- Try booting from the backup volume again.
If you see the universal "No access" symbol after selecting your startup disk
This indicates that the kernel cannot load the kernel extension cache, or that it cannot mount the startup disk. This could be due to trying to run an incompatible operating system on your Mac, due to an extension conflict with the enclosure you are trying to boot from, or due to a firmware compatibility problem between the Mac and the enclosure.
We see this quite frequently when trying to boot from a USB 3.0 enclosure, especially on pre-2013 Macs. 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.
- Try booting into Safe Boot mode (hold down the Shift key as you start your Mac, after selecting the backup volume as the startup disk).
- Try installing macOS directly onto the cloned volume while booted from the Apple Recovery volume. 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 blue screen 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 cloned volume while booted from the Apple Recovery volume 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 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.
"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. We have found a few conditions that will prevent you from making a startup disk selection:
APFS is not a supported, bootable format on older OSes
If you boot from an older backup, e.g. macOS Sierra, APFS-formatted volumes may mount in the Finder, but you may have trouble selecting them as a startup disk. APFS was a beta filesystem on Sierra; the components required for making an APFS volume bootable were not yet baked.
Likewise, support for APFS-formatted Fusion volumes was not added until macOS Mojave. If you boot from a High Sierra backup volume, you'll notice that your APFS-formatted, Mojave-containing Fusion volume is mounted read-only, and you will be unable to set that volume as the startup disk.
The solution in both cases is to use the Startup Manager (boot your Mac while holding down the Option key) to select an alternate boot disk. Once you have booted your Mac from the newer OS, you will be able to reset the startup disk selection.
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. Multiple, bootable volumes within a single APFS container is a supported configuration, but you can only make the second volume bootable if you boot from some other startup disk for the duration of the cloning procedure. Likewise, you will be unable to change the startup disk selection to the second volume while booted from the first volume. The solution is the same as above — use the Startup Manager (boot your Mac while holding down the Option key) to temporarily change the startup disk selection, then set the startup disk explicitly to the new startup volume.
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.
- Open Disk Utility
- Choose "Show all devices" from the View menu
- Click on the top-most parent device for your Macintosh HD volume
- Click the "Partition" button in the toolbar
- When Disk Utility tries to discourage you from doing this, click the "Add partition" button
- Click the "+" button to add a second APFS-formatted partition on the startup disk
The bless utility cannot bless some Firewire-attached devices
We have received a handful of reports that attempting to select a Firewire-attached volume as the startup disk yields this same "unable to bless" error. In the cases where USB was an alternative option, selecting the device as the startup disk works fine when connecting the same device to the Mac via USB.
- 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.
- Some Western Digital enclosures cannot boot Macs. That's a very old article that does not appear to be getting regular updates from WD. More recently we have received a report 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).
- Hands Off! and possibly Little Snitch may prevent a cloned volume from booting
- Some Macs have trouble booting from USB 3.0 enclosures
- Some MacBook Pros can boot from a disk in an enclosure, but not when it is installed internally
- Some MacBook Pros can boot from a disk in an enclosure, but not when it is installed internally (asks to be reinitialized)
- Cannot boot from a hard drive installed in some optical bay replacement kits
- Reports indicate that, contrary to its published documentation, the NewerTech Voyager Dock enclosure is not bootable via Firewire.
- 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 IOCrest adapter and 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".
Some users have reported that the Samsung T5 Portable SSD cannot function at all as a bootable device on the T2-based MacBook Pro 2018. Efforts even to install macOS Mojave onto this device fail to produce a bootable volume. This is a popular enclosure that we've seen great success with, and so far these reports are limited to the 2018 MacBook Pro.
The Samsung T5 Portable SSD (and also the Transcend StoreJet SSD) also introduces an exceptional delay during startup (on any Mac, not just T2 Macs), whether you're attempting to boot from that device or your Mac's internal hard drive. This appears to be a compatibility problem between the Mac's firmware and this particular SSD when the SSD is formatted as APFS and when the SSD has an installation of macOS (whether placed there via cloning or via the Installer). To avoid this delay, we recommend formatting these SSDs as HFS+ until the compatibility problem is resolved:
- Open Disk Utility
- Choose Show all devices from the View menu
- Select the top-level "parent" device of the Samsung T5 SSD in Disk Utility's sidebar
- Click the Erase button in the toolbar
- Set the format to Mac OS Extended, Journaled, set the Scheme to GUID Partition Map. and give the new volume a name
- Click the Erase button
- Open CCC and re-select the new volume as the destination, then run the backup task
Note: If you have a T2 Mac, please bear in mind that T2 Macs cannot boot from an encrypted HFS+ formatted device. The Samsung T-series devices will not be a suitable backup device for your T2-based Mac if you require that the backup disk is encrypted.