There is a question which comes up quite a bit lately related to not having a bootable source. It's a kind of "Catch 22" situation where you need to be able to boot in order to fix an issue that is preventing you from booting.
Easily solved if you have a bootable clone or a USB thumb drive macOS installer but commonly this is not the case.
If you have a recent Macintosh, you can simply start up from Recovery Over The Internet:
About macOS Recovery - Apple Support
Time Machine offers an option that not many OP's seem to be aware of; it has a backup of the Recovery Partition. You can boot from that and use it to access Disk Utility. So even the bog standard TM backup could save your *** in the case of a system failure. This fact and the steps involve could be valuable info for an FAQ.
Ever since OS X 10.7.2, Time Machine backups are bootable. They won't boot into the version of the OS that you were running, and you can't log in to your account, but they will boot into the equivalent of your Recovery Partition. From there, you can reformat disks and do a full restore.
As for backups, it is typically a case of either using the free program Time Machine, or using Carbon Copy Cloner or SuperDuper!. I actually read with interest the way one can create a clone via Disk Utility. In fact, I never knew about that.
There is the ability to create a clone backup built right into the Mac OS. But it is quite a bit slower than using CCC or SD. Another huge disadvantage to it is that there in no ability to do a smart update. If your data on your main hard drive changes, you have to do a complete new clone backup from scratch if you want to add your new data to your backup. However, the ability to create a clone backup built into the Mac OS has the huge advantage of being free. Here is how you do it:
- Plug the backup HD into the computer.
- Launch Disk Utility.
- Select the HD you want to copy in the column to the left.
- Click on the Restore tab.
- That HD you just selected should appear after Source:
- From the window on the left, drag the icon of the HD to which you want the Source HD copied.
- You should see a note with something to this effect: Erase the Destination HD and copy the contents of the Source HD to it?
- Click on the Restore button and that’s it.