Glossary of Terms

This documentation is for an older version of CCC. You can find the latest version here.
Last updated on February 26, 2021

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Apple File System (APFS) — APFS is a new filesystem introduced by Apple in macOS High Sierra as a replacement for the legacy HFS+ filesystem. See also: Everything you need to know about Carbon Copy Cloner and APFS

Apple Filing Protocol (AFP) — AFP is a file sharing protocol that allows you to access the files on other computers and NAS devices on your network. CCC can copy files to and from folders and sharepoints on SMB and AFP sharepoints. AFP is deprecated in favor of the SMB protocol starting with OS X Yosemite.


Backup — A backup, or the process of backing up, refers to the copying and archiving of computer data so it may be used to restore the original after a data loss event. The verb form is back up, in two words, whereas the noun is backup. In other words, you back up your data using CCC. When you have done that, you have a backup of your data on physically disparate media.

Bootable backup — Same as backup, but a backup of a volume that contains an operating system that can be used to boot the computer if the primary startup volume fails.

Boot selector — See Startup Manager.


Checksumming or "Find and replace corrupted items" — With this option, CCC will calculate an MD5 checksum of every file on the source and every corresponding file on the destination. CCC then uses these checksums to determine if a file should be copied. This option will increase your backup time, but it will expose any corrupted files within your backup set on the source and destination. This is a reliable method of verifying that the files that have been copied to your destination volume actually match the contents of the files on the source volume.

Clone — A copy of a folder or volume; a non-proprietary backup. While not identical (some caches should not be copied as they must be rebuilt on a bootable backup, and files like trash are excluded), clone is a common word used for a CCC bootable backup.

Container (APFS) — A container on an APFS formatted drive is similar to a partition, but allow several volumes to share the space in the container more flexibly. See: Working with APFS Volume Groups

Cruft — Another term for digital detritus, e.g. files that could (should) be deleted because they're no longer needed nor desired by the user. This term was coined to describe the large collections of technical equipment piled in the corridors of the Cruft lab at MIT in the 1980s and 90s.


Destination — The location where files from the source are copied. The destination can be a disk attached directly to your Mac, a network location (e.g. a NAS or a share from another computer), or a disk image file. Destination is a relative term. When making an ordinary backup, the destination is your backup volume. When restoring, however, the destination is your original volume, or a replacement device.

Differential backup — A differential backup is a type of data backup that preserves data, saving only the difference in the data since the last full backup. CCC uses a differential backup method, but does not store the differential data in a proprietary manner. Rather, the files are copied to the destination among the already-up-to-date items such that the destination is a clone of the source.

Disk image — Disk images are data containers that emulate disks. When you open a disk image file, a virtual volume is mounted that allows you to browse the files held by the disk image – as if you were browsing a physical disk device. Disk images are recommended only when backing up to a network destination to protect attributes that are not supported by the network volume. Disk images are not bootable. Backing up to a disk image

  • Sparse bundle disk images appear as a single file but are actually folders with many files inside. This is more efficient, as only the changed parts need to be copied when updated.
  • Sparse disk images are stored as a single monolithic file. This can make backups less efficient as the entire file must be copied each time. 


EFI Partition — The EFI partition is an Apple-proprietary partition. That partition is created automatically when a disk is partitioned with the GUID partition scheme, and its contents are managed internally by OS X. Third-party applications shouldn't attempt to modify, nor copy that volume.

Extended Attribute — Extra data that is associated with a file. Extended attributes typically contain non-user-created data that was placed there by the application that created the file. For example, photo applications may place thumbnail icon data into an extended attribute. CCC attempts to copy extended attributes when possible, but extended attribute data is generally considered to be disposable because it can be regenerated by the application that created it. Advanced Settings: Don't preserve extended attributes


Filesystem, or file system — A volume's filesystem controls how files and folders on that volume are stored and retrieved, and also controls who can access those items.

FileVault Encryption — Volume level encryption built into the macOS. When enabled on a volume, a password is required to unlock and mount that volume. Unlike ownership-based restrictions, FileVault protection persists when attaching the disk to another computer.
Apple Kbase #HT204837: Use FileVault to encrypt the startup disk on your Mac

Firewire — Firewire is an interface standard developed by Apple that allows the connection of external peripherals to a computer. Firewire devices provide reliable bootability and excellent performance that rivals USB 3. This interface has largely been supplanted by Thunderbolt on newer Macs.

Firmlink — A firmlink is described by Apple as a "bi-directional wormhole" between two filesystems. A firmlink transparently redirects the navigator from a read-only folder on a System volume to a writable folder on a Data volume. These are similar to aliases, but they are only applicable to folders, and they cannot be created by the user.


HFS+, or "OS X Extended, Journaled" — The default filesystem format used for macOS system volumes. First introduced for Mac OS 8, HFS+ has been updated for many years to support new features of macOS. Apple introduced a replacement for HFS+ in macOS High Sierra: Apple File System.


Incremental backup — An incremental backup is one that provides a backup of files that have changed or are new since the last backup; it is one that backs up only the data that has changed since the last backup. When making a backup for the first time, an incremental backup copies all files.


Migration Assistant — A tool from Apple that allows you to migrate applications, settings, and documents from a backup or older computer to a new computer or fresh installation of the OS. You can use a CCC bootable backup as a source for Migration Assistant.
Apple Kbase #HT204350: How to move your content to a new Mac


Network Attached Storage (NAS) — NAS systems are networked appliances (e.g. a router or a specialized storage device that connects to your router) that contain one or more hard drives. They typically use SMB add/or AFP networking protocols to make sharepoints available to macOS, Windows, and Linux clients.


Partition — In verb form, partition refers to the process of creating a division on a hard drive that defines one or more volumes. When you purchase a new hard drive, it often must be partitioned to make it suitable for use on your Macintosh. In noun form, partition is colloquially used in the same manner as a volume. A partition table refers to a hidden structure on a disk that defines the size and position of the volumes on a disk. CCC does not copy the partition table, nor multiple partitions on a disk. Rather, a CCC backup task is defined with one source volume and one destination volume. Preparing your backup disk for a backup of OS X

Prune — Remove older, archived material that was cached on the destination volume. Automated maintenance of the CCC SafetyNet folder

Permissions — A file and folder specification that defines the access that various users and groups will have with regard to reading or modifying that item.

Preflight/Postflight script — An advanced feature; shell scripts that can be added to the beginning or end of a CCC backup task to extend the task's functionality. Running shell scripts before and after the backup task


RAID ("Redundant Array of Inexpensive Disks" or "Redundant Array of Independent Disks") — A collection of hard drives that using software or hardware are presented as one or more volumes. There are several levels of RAID that balance speed and redundancy. See this Wikipedia article for more details.

Recovery HD — A hidden, Apple-proprietary volume associated with a macOS startup volume. The Recovery HD offers a method to reinstall macOS, and also must be present prior to enabling FileVault encryption on the associated startup volume. The presence of a Recovery HD volume is not required for maintaining a bootable backup of your startup disk, nor for recovering from a bootable backup. Cloning Apple's Recovery HD partition

Root — the root folder (also known as the root directory) is the first or top-most folder in a hierarchy. When you double-click on a hard drive icon in the Finder, the folder that appears first is the root-level folder.


SafetyNet — A feature in CCC that protects files on the destination from being accidentally deleted. If you have files on your destination device that don't exist on the source, those files get placed in the SafetyNet. CCC will also place the older version of modified files into the SafetyNet. The SafetyNet is a temporary safe haven for files unique to the destination. When space is constrained on the destination, CCC will start to remove older items from the SafetyNet. Protecting data that is already on your destination volume: The Carbon Copy Cloner SafetyNet

Seed — Initially populating a destination volume while it is attached directly to your Mac. This "seeded" volume can then be attached to a remote Macintosh at a distant location, and subsequent backups will be faster because less data will be copied over the Internet.

Server Message Block (SMB) — SMB is a file sharing protocol that allows you to access the files on other computers and NAS devices on your network. CCC can copy files to and from folders and sharepoints on SMB and AFP sharepoints.

Shell Script — A text file containing command-line arguments that can automate tedious tasks. CCC backups can be configured with pre and postflight shell scripts to extend the functionality of the backup task. For example, you could implement a postflight script to unmount the source volume. Running shell scripts before and after the backup task

Sidebar — An interface element that appears on the left side of CCC's main window when you click the Show Sidebar button in CCC's toolbar. A table at the top of CCC's sidebar lists your CCC backup tasks, while a table at the bottom of the sidebar lists all of the locally-attached volumes that are currently mounted on your Mac. The contents of the sidebar are also accessible via CCC's View menu.

Simple Mode — A simplified user interface. Simple Mode significantly reduces the number of user interface elements — the sidebar, toolbar, scheduling selector, and advanced settings are all suppressed, leaving the user with only three primary controls: Source, Destination, Clone button. Simple Mode

Snapshot — A snapshot is a recording of the state of a system at a particular point in time, an analogy to a photograph. You can restore your system to a prior point in time using a snapshot.

Source — The folder or volume that holds the data that you want CCC to copy.

Span — When a backup extends past a destination for more room. CCC does not support spanning multiple destinations.

Sparse file — Sparse files consume less space on disk than their file size would suggest. Sparse files are occasionally used for log files, databases and virtual machine files. CCC can preserve sparse files between APFS volumes, but HFS+ does not support sparse files, so these files consume more space on an HFS+ formatted backup disk.

Startup Manager — A system tool from Apple that allows you to select a startup volume as the Mac is starting up. The Startup Manager is part of your Mac's firmware; hold down the Option key while turning on your Mac to bring up the Startup Manager.
Apple Kbase #HT204417: How to select a different startup disk


Target Disk Mode — An alternate startup configuration in which the computer does not boot to the loginwindow nor Finder. Rather, a Firewire, USB, or Thunderbolt icon appears on the Mac's screen, and when you attach the Mac to another Mac via Firewire, USB or Thunderbolt, the internal storage of the Mac in Target Disk Mode appears on the Desktop of the other Mac. In other words, Target Disk Mode makes your Mac behave like an ordinary external hard drive enclosure.
Apple Kbase #HT201255: Mac startup key combinations

Task — A collection of settings in CCC that define a source, destination, items to be copied, and automation.

Task chaining — A feature in CCC that allows you to run another task at the end of a task, see: Performing actions Before and After the backup task: Run another backup task (task chaining).

Thunderbolt — Thunderbolt is a hardware interface developed by Intel that allows the connection of external peripherals to a computer. Thunderbolt is a popular, albeit pricier interface for connecting external hard drives to your Mac. Thunderbolt devices provide excellent performance and reliable bootability.


Universally Unique Identifier (UUID) — A 36-character hexadecimal code (characters A-F, 0-9) that uniquely identifies a volume, e.g. "F5B1D7B0-66EC-4082-A34C-86FFD294FA61". When you erase a volume with Disk Utility, the new volume gets a new unique identifier. CCC uses this identifier, along with the name of the volume, to positively identify the source and destination before copying any files. Due to the unique nature of these identifiers, they prove more reliable than volume name when identifying a volume, because there's nothing stopping you from naming all of your disks "Macintosh HD".

Universal Serial Bus (USB) — An industry standard for cables, connectors, and communication between a computer and some external devices like a hard drive, keyboard, or mouse. Macs and USB devices can adhere to the USB 2 or USB 3 versions of the protocol, depending on when the device was manufactured. USB 3 is considerably faster than USB 2. Macs produced before 2012 do not have native support for USB 3. USB 3 devices can be used with those Macs, but will be connected at USB 2 speeds.


Volume — The terms "disk" and "volume" are often used interchangeably. Ambiguity arises, however, when you modify the partitioning of a disk such that it has multiple volumes. The term "disk" refers to the physical, whole device. A disk contains volumes, and its a volume that you see in the Finder (frequently with a hard disk icon, bringing the confusion full circle). A helpful graphic is available in this section of CCC's documentation.