How to create a boot clone
Overview—
Part of a prudent backup plan is having a Boot Clone. This along with an independent Time Machine (TM) back-up will help save your bacon in most any type of data lose, system corruption, catastrophic failure or reinstalling an Operating System. The key concept in any back-up plan is redundancy, (i.e. more than one single back-up device and more than one methodology) the boot clone can serve as a vital part of this one two punch of data protection.
All Hard Drives ( read HD, SSD, SSHD [solid-state hybrid drives] or Fusion drives ) will fail, and can do so at the most inopportune time. One of the main benefits of the Boot Clone is you can boot your machine directly from this external HD, and pick up where you left off without experiencing any down time in your productivity should your internal HD fail. This will allow you to come to terms with your current situation and make a contingency plan for repair, all the while having a fully functional macOS to continue operating.
Time Machine does not have this capacity to boot as a usable macOS, however according to Apples support article (How to set up and use an external Mac startup disk - Apple Support) you can select your TM backup drive (”EFI Boot”) from the Startup Manager and it will boot into Recovery Mode—from which you can use the Disk Utility to format a replacement drive—and subsequently use TM to restore its content.
The main advantage of TM is that it creates recursive backups—enabling you to restore a specific file or the entire drive— from a certain point in time.
In addition, TM keeps local snapshots on your HD, space permitting for a 24-48 hour time period.
These local snapshots are automatically enabled when you turn on Time Machine. About Time Machine local snapshots - Apple Support
If your Time Machine (Time Capsule) fails, becomes corrupt or does not inherit—you will be thankful you had an alternative backup strategy.
If your Mac is "user up-gradable" with a new internal HD/SSD you can format this new drive externally, and create a boot clone to this drive before removing and replacing your internal HD. This is a fast and easy process, and a simple enough procedure if you have the correct software, two of the most popular:
Carbon Copy Cloner https://bombich.com/
Super Duper http://www.shirt-pocket.com/SuperDuper/SuperDuperDescription.html
You will need an external enclosure to hold the new drive. There are many options, a popular choice is OWC ( now MacSales.com) https://eshop.macsales.com/search/enclosure Here you can buy HD/SSD replacements, enclosures or kits (with tools included https://eshop.macsales.com/Search/?Ntk=Primary&N2=4294931955&Ns=P_Popularity%7c1 &Ne=4294933107&N=4294931955&Ntt=DIY+KIT+… )
It is always recommended if replacing a SATA drive, to go ahead a replace the SATA cable at the same time. (See OWC)
First all new drives must be formatted using Disk Utility.app—this is found in your Applications/Utilities folder, or via Recovery http://support.apple.com/kb/HT4718 or from a bootable USB installer https://support.apple.com/en-us/HT201372
The important part of formatting your drive for macOS is a GUID Partition Table and the file system Extended Journaled. Verify from Disk Utility>View>Show All Devices (you want to be erasing/formatting the "Parent" drive, not a "child.")
Always best to let the installer do the conversion for you to the new Apple File System (APFS) to avoid problems.
Some instances the Catalina installer.app is looking for APFS before the install.
You can read more:
About the read-only system volume in macOS Catalina - https://support.apple.com/en-us/HT210650
Working with APFS Volume Groups - https://bombich.com/kb/ccc5/working-apfs-volume-groups
Hopefully this short overview demystifies the concept of Boot Clones, the advantage of a redundant backup plan, and the resources to help you move forward. Whatever you do, back up early and often.
