The operating system has very specific requirements to be able to boot off of a disk. This document describes how to fix problems found when setting up a bootable backup, as described in Bootable Backup Script Template.
All of these operations take place using /Applications/Utilities/Disk Utility.app.
IMPORTANT: Repartitioning or reformatting permanently removes all information currently on the backup disk. Make sure you don't have anything you want to keep on the backup disk before continuing.
To be bootable, a disk must be partitioned with a “GUID Partition Table” and the volume formatted as “Mac OS Extended (Journaled)”.