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Disk cloning is one of many possible measures you can take to prevent data loss. A disk clone is an exact duplicate of the entire volume (or entire disk if there is only one partition). If the original disk is bootable, its clone is also bootable. This means that you can backup not only your documents but also the operating system. The duplicate of your disk can be used to boot your computer.
External drives such as HDD or SDD are the best media for a bootable disk clone.
The process of disk cloning takes quite a lot of time. Usually it isn't convenient to clone your disk frequently. Subsequent runs of the disk cloning take less time because the program updates only the modified or removed files. Planning your backup strategy, think about a combination of disk cloning with regular backups (e.g. incremental or full) that include only user files. In such case, backing up can be more frequent (for example, daily), and disk cloning less frequent (weekly or monthly).
If you plan to use your disk duplicate as an emergency working system, the disk for a clone should be of sufficient capacity. It should be about 10% larger than all of your files take on the original disk.
It is recommended that you run the Disk Utility application to fix possible file system problems. The tools that allow you verify your disk are located in the First Aid tab. This application is in the Applications/Utilities folder.
The disk cloning tool erases everything on the destination disk. Make sure that there are no critical files on it.
This documentation supposes that you create a system backup to be able to boot from it on the same computer.
It is recommended that you boot from your emergency disk to make sure that a copy of your operating system works well.
There are two ways to specify from what disk to boot your Mac. If your current operating system is running, click the Apple menu and select System Preferences…. Choose a disk in the Startup Disk section. If you cannot boot from the regular disk or your computer is turned off, press and hold the Alt (Option) key before turning power on.
Before starting cloning, Get Backup turns off the Spotlight indexing for the destination drive. If you need to turn it on, use the Privacy settings in the Spotlight section of the System Preferences.
Disk cloning requires the APFS or Mac OS Extended (HFS+) file system. This file system has several subversions such as Encrypted or Case-sensitive. It is recommended that the destination disk has the same format as the source disk. Many disks and USB flash drives have been formatted as FAT32. You can copy files on these disks but cannot use as a start up disk until it is properly formatted.
|Mac OS Extended||Mac OS Extended||Yes|
|APFS||Mac OS Extended||Yes|
The partition scheme should be GUID Partition Table (GPT).
The destination disk should be writable. If it has any write protection, deactivate it.
The Partitioning and Formatting your Disk section gives you an idea of how to prepare a disk for creation a bootable copy of OS X.