The question:

is it possible to keep a boot-able backup drive clone of OS X updated with rsync? If rsync is not a viable option are there alternatives?

The Setup:

My situation is as shown above. One internal Samsung 840 SSD [120g] in use as my OS X 10.8 boot disk on a recent model Mac Mini. I have successfully cloned that drive with disk utility to a 125g partition of another HDD in an external USB 3 enclosure and at that point I am able to boot to it.

The Goal:

As my last system went out in a fiery blaze taking much valuable data with it, I have a new respect for a proper backup solution and really want to do this right. My goal is to achieve an automated differential backup/update from Disk A to Disk B while most importantly maintaining boot-ability on the external drive. And I would prefer to do this differentially to minimize stress on the drives. Hence rsync was the first thing to come to mind.

What I have tried:

following along with Jamie Zawinski's differential mac bootable backup solution

running this manually initially worked - i tested it with only very miniscule file change and everything was fine / external booted and all. now after subsequent passes rsync fails throwing errors particularly relating to updating 'boot.efi' (not at the machine currently I will update the precise log message once I return home)

is this a drive partition size issue? does rsync require more space?

if it cant be done, are there any alternatives? i've heard whispers of dd