If you're looking to clone a hard drive bit-for-bit, nothing beats Unix/Linux DD. It's built into every version of Linux/Unix/BSD I've ever used, including Mac OS, and you can find many Linux "live" discs that have it.

Of course, there are a few caveats. You end up copying all the garbage along with it, and your target drive has to be exactly the same size in bytes or bigger than the original. Also, it's very slow, and there is no intelligent updating.

I personally use SuperDuper and Time Machine on Mac, or Clonezilla for everything else.

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