While Time Machine makes incremental backups that let you go back into time to look for files, a full bootable backup takes a snapshot of your hard drive at the time you make the backup. Both kinds of backup are important and if you can make both then you are doubly protected.
A full bootable backup is useful if you have a hard drive crash or lose your Mac or it is stolen. Read about my MacBook's hard drive crash. From a bootable backup, you can quickly restore all of your lost data since the last backup. Unfortunately, Apple does not come with software to create a full bootable backup.
For a full bootable backup, I recommend SuperDuper! It is straightforward and easy to use. SuperDuper! costs $27.95US which is quite a bargain for what you get. You can learn more about it here.
Note that a bootable backup requires another external hard drive besides the one you use for Time Machine. Unlike Time Machine, however, this external drive only needs to be connected when you are making the backup. The rest of the time you can keep the drive in a safe place.
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