A closer look at how CCC determines the “bootability” of a destination volume

This documentation is for an older version of CCC. This article has been retired, so it does not exist in the knowledge base for the newest version.
Last updated on October 22, 2019

CCC determines whether your destination volume will be bootable and indicates any configuration concerns in the "Cloning Coach" window. If you see a yellow warning icon in the Task Plan header, you can click on that icon to see these concerns. CCC will also present these concerns to you the first time that you configure a backup task to any particular destination volume.

If CCC doesn't raise any configuration concerns, and the destination volume has an OS on it when the backup task is completed, and barring any hardware problems that might interfere, your backup volume should be bootable.

Configuration concerns that affect the bootability of the destination volume

CCC looks for the following configurations to determine if a destination volume will not be bootable:

  • The destination volume cannot be a disk image — you cannot boot your Macintosh from a disk image.
  • The files and folders required by macOS must be present on the source volume. These include: /Library, /System, /bin, /etc, /mach_kernel, /private, /sbin, /tmp, /usr, and /var.
  • The files and folders that are required by macOS must not be excluded from the backup (applicable only if you have chosen to back up Some files).
  • The hard drive on which the destination volume resides must be partitioned using the GUID Partition Table partitioning scheme.

Related documentation:

"Some file metadata cannot be preserved"

CCC will note a concern if there is a compatibility mismatch between the source and destination filesystems. For example, if you are backing up files from an HFS+ volume to a network filesystem, some of the filesystem metadata cannot be preserved. In many cases this is acceptable and you can ignore the message. The types of metadata that can't be preserved in these cases are described in more detail below.

Access Control Lists

Access Control Lists specify a granular list of the privileges that users and groups have for a particular file or folder (e.g., read, write, get information, delete, etc.). These advanced privilege settings generally apply only to user accounts that have been created on your Macintosh — for example, to prevent other users from deleting items from your home directory. If you are backing up your own files to a locally-attached hard drive, or to a network file share on a trusted computer, the Access Control List filesystem metadata is relatively unimportant. If you are backing up to or from a network filesystem in a business or education setting, however, check with your tech support staff for additional advice on whether this metadata must be preserved.

Hard links

A hard link makes a single file appear to be located in multiple places on your hard drive. If a single file had 20 hard links scattered across the disk, each hard link file would consume no additional space on the hard drive, and editing the content of any one of those files would immediately affect the content of every other hard link to that file.

When you back up the contents of a volume that contains hard links, ideally you want to preserve the hard links. If the destination filesystem doesn't support hard links, each hard linked file will be disassociated from the original file and will become a copy on the destination. This won't result in any loss of data, but your backup set will consume more space on the destination than on the source. Hard links are leveraged quite a bit on macOS by the operating system, though they are generally less common among user data.


File ownership indicates which user account on your Mac has control of a file. The owner of a file can limit access to that file from other users on the same computer. If the destination doesn't support ownership, then the owner of each file copied to the destination will be set to the user that mounted the destination. If the destination volume is accessed elsewhere (e.g. mounted on another Mac or even by a different user on the same Mac), then any restrictions that you have placed on those files may not be honored. If you are backing up files and folders that are not all owned by the same user (e.g. you), you should consider backing up to a local, HFS+ formatted volume or to a disk image instead.

Some filesystems have file size limitations

Some filesystems have restrictions on how large a file can be. FAT32, for example, limits files to 4GB or less. CCC will proactively warn you of this limitation if you choose to back up a volume whose filesystem supports files larger than 4GB to a filesystem that does not support files larger than 4GB. CCC will then automatically exclude files larger than 4GB from the backup task. Files that were excluded will be reported at the end of the backup task.

If you require that files larger than 4GB are backed up, you must reformat the destination volume with a format that supports larger files.

Related documentation:

The destination already has an installation of macOS. Merging a different version of macOS into this destination may cause problems with that installation of macOS

This message appears if you choose the Don't delete anything SafetyNet setting. While that setting will protect any data that you have on the destination volume that is unique to that volume, it does a disservice to the installation of macOS on your destination. This message will also appear if you use the Don't update newer files on the destination advanced troubleshooting setting.

Suppose, for example, that you have a complete backup of Mac OS 10.12.4 on your backup volume. When you apply the 10.12.5 update to your source volume, many system files are updated, some new files are added, and some files may be deleted. If you use CCC to update your backup volume, but you don't allow CCC to delete the items on the destination that the OS update had deleted from the source, then there will be a bunch of "cruft" left over on the backup volume. If you should ever need to boot your Mac from your backup volume, these cruft files could cause the OS to behave unexpectedly, and they may prevent it from booting altogether.

CCC can help you perform a clean upgrade or downgrade of macOS on the destination volume by moving items that should be deleted to the SafetyNet folder. Any files and folders that you keep only on the destination would also be moved to the SafetyNet folder. See the Protecting data that is already on your destination volume section of the documentation for more details on these settings.

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

In the past we received several reports of bootability problems related to USB devices larger than 2TB. At that time, we performed a simple litmus test: create an "x"TB partition at the beginning of the disk (varying x from 0.5 to 2.5TB) and a second partition consuming the remainder of the disk, then install macOS onto both partitions. The results of those tests suggested that some Macs couldn't "see" the partition that lied past the 2TB mark on the disk. This limitation was specific to USB devices — none of these problems occurred if you were to place the same disk into a Thunderbolt enclosure.

At the time of those initial reports and testing, the results were consistent. We concluded that there was likely a 32-bit addressing limitation imposed by the USB drivers that are embedded in the Macs' firmware ("likely" — unfortunately none of this information is documented by Apple). More recently, however, we've been unable to consistently reproduce the same results. Apple may have addressed the problem with a firmware update. It's also possible that our initial conclusion was wrong, e.g. that the problem was due to a partition alignment error; an error specific to macOS El Capitan and apparently only USB devices (you'd see "disk2s2: alignment error" messages in the system log when the affected volume is mounted).

In any case, CCC's warning was issued out of an abundance of caution. Our current recommendation is to partition the destination device using the same procedure as defined for all other destination devices, and do the partitioning while booted from any other OS than El Capitan. In other words, don't proactively create a 2TB partition at the beginning of the disk. Once you have completed your first backup, though, we encourage you to verify that your Mac will boot from the backup volume. If your Mac is unable to boot from the backup volume, please reach out to us so we can investigate your specific configuration further.

Help! My clone won't boot!

See this section of CCC's documentation for troubleshooting advice if you're having trouble getting your backup volume to start your Mac.