Carbon Copy Cloner, v4.1.13 (been in business since 2002)
MSRP $40.00
System Requirements: OS X 10.8 through 10.12.3

The purpose of CCC is make bootable backups (full or incremental) on any schedule you desire.

Parenthetical: The developer of CCC, Mike Bombich, is himself a clone. Well, an identical twin…

In the spirit of full disclosure, For several years I used the principal competitive product, Super Duper! (SD), and always found it first-rate; both from a technical and support perspective.

So why the change?

Well, the simple answer is that with the passage of time, and as my computer skills increased, I appreciated the extra capabilities CCC provided. But make no mistake, there are a few minor disadvantages. And these are noted on the last page.

But before I go too far, be advised, comparing CCC with the competition harkens to “Printer Wars”. Some/much of your decision (CCC or SD!) will boil down to what you, the user, prefer. But as for moi, I find CCC superior to SD! Of course, YMMV!

Downloading and installing CCC is easy-peasy and the set-up is quite intuitive. And if you have questions, their tech support responds within a day or two; often within hours.

CCC supports cloning of the Recovery Partition (SD! does not).
Can customize to NOT copy selected files.
If (during a backup) CCC encounters a corrupted file or files, it will "keep on going", trying to clone all the “good” files, then give you a report when it’s finished. (SD! would abort this cloning.)
It is ideal for all levels, from beginner to genius.
I find that cloning back required less “cleanup” than does SD!
CCC respects Energy Saver Settings (laptops).
Preferences & settings are easy to understand.
There are both Simple and Advanced settings.
There is a Safety Net feature that allows archiving those files replaced/updated in the target drive. Users can determine how much, if any, space is dedicated to this feature.
Can copy a Windows partition (but it won’t be bootable),
Free 30 day trial, try before buy.
License is for every Mac in your household.
Easy to edit tasks and their schedules.
Easy to customize backups, run scripts, etc.
Has a shell script to unmount backup disks upon copy so malware cannot attack your backups.
It’s a perfect compliment to Time Machine.

Clone history cannot be cleared per target volume. Rather, clearing the history clears all CCC history.
SuperDuper! has a SandBox feature which is very nice for software testing; CCC has no such feature.
SuperDuper! will backup a Time Machine, CCC will not.
SuperDuper! is $12.00 cheaper.

Cloning time, via CCC or SD! is essentially identical.
I know of no drive size constraints.
I have no reservations in recommending CCC to any/all users, regardless of their skill level.
Without doubt, CCC is a 5 star application!

Not all hard drive enclosures are capable of booting macOS. Check with the manufacturer of your hard drive enclosure to verify that booting from the enclosure is explicitly supported.