CCC includes a command line utility that allows you to start, stop, and monitor the progress of specific CCC backup tasks. The utility is located inside of the CCC application bundle. To get basic usage instructions, invoke the utility without arguments in the Terminal application, e.g.:
For improved detachability, macOS will unmount any non-internal volumes that are attached to the system when you log out. So, for example, if you log out of your computer while a USB or Thunderbolt hard drive enclosure is attached, you can detach those hard drive enclosures from the system without having to manually unmount them first. This is a good thing — it would be annoying if you had to log back in to your system just to eject a drive. The downside of this, though, is that if you have a CCC backup task that runs when no user is logged in, the destination volume may be unavailable.
If you upgraded your Mac to macOS Mojave and have decided to downgrade for one reason or another, the procedure is usually pretty straightforward. Fusion volumes, however, introduce a complication. Upon upgrading to Mojave, a Fusion volume will be converted from HFS+ to APFS. If you want to downgrade to High Sierra (or any earlier OS), you must reformat that Fusion volume as HFS+. Because APFS Fusion volumes are not handled gracefully by High Sierra, however, the procedure is a bit tedious.
At its core, Carbon Copy Cloner is a product that is designed to make bootable backups of your Mac's operating system. In order for CCC to be able to make copies of system files, CCC needs to have the privilege of copying files that can't be read nor written by just any user – CCC requires elevated privileges to copy macOS system files. Likewise, CCC is often tasked with copying the data associated with multiple users. macOS prevents you from accessing files that belong to other users.
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.