Is the backup drive also the boot/startup drive?

[cgm707]

After reading the help section, I am still uncertain about creating a backup boot/startup disk.

Does the external backup disk also serve as a backup startup disk? Or do I use a different external drive? If it's a different drive than my backup drive, which instructions do I follow?

 

[gsahli]

From here - https://support.apple.com/en-za/HT201250

In OS X Lion v10.7.3 or later, you can start up from your Time Machine disk, if necessary. Hold down the Option key as your Mac starts up. When you see the Startup Manager screen, choose “EFI Boot” as the startup disk.

 

[Fishrrman]

To create a BOOTABLE backup drive you need the following:

1. An external drive (USB3 will do fine).
2. Either CarbonCopyCloner or SuperDuper (both are FREE to download and try for 30 days).

Then...
1. Initialize the backup drive to HFS+ with journaling enabled
2. Launch CCC or SD (I prefer CCC)
3. Put your source drive (internal) on the left. Put the target (external) to the right. Choose "safety net OFF". Go!

When done, to BOOT FROM the cloned backup, do this:
1. Restart the Mac
2. IMMEDIATELY hold down the option key, and KEEP HOLDING IT DOWN until the startup manager appears
3. Select the external backup with the pointer and hit return
4. The Mac should now boot from the cloned backup
5. When you get to the finder, it will LOOK EXACTLY THE SAME as does your internal. To verify that you are booted from the backup select "about this Mac" (Apple menu), and check which drive is the boot drive.

You didn't say whether your internal (on High Sierra) is formatted to HFS+ or APFS.
I believe CCC (possibly SD as well) now can create an APFS bootable backup (as well as an HFS+ bootable backup).

A bootable cloned backup will serve you better in a "moment of extreme need" (such as "I can't boot!") than will a TM backup.

 

[laurihoefs]

Can you clarify a bit what you want to achieve? There are two good answers in this thread already, but they describe somewhat different solutions.

Do you mean you want a fully functional backup copy of the OS and your files on an external drive, that you can then boot up and use just like the internal drive?

Or do you just need a way to start the recovery process from an external drive?

The former requires the steps Fishrrman listed, the latter is already possible with Time Machine, like gsahli wrote.