Hello and welcome to my User Tip
This is a step by step instruction how to make and boot a bootable clone of your OS X system.
It can be used for backup, moving to a larger drive, moving your users to a new Mac (using Migration Assistant), defragmenting and optimizing the system (with full clone), shifting data up on hard drives to make (more) room for BootCamp or another partition or as a data recovery/undelete boot drive.
Made before disaster strikes, it's a real lifesaver!
See these other related User Tips.
Getting the right external drive
Get a external hard drive equal to or larger than the drive you intend to replace the internal one with. (if so) So later it will be bootable backup (clone) of that new drive.
If you like the idea of saving the differences between the clone updates, then get a much larger drive. If maintaining a pure clone, then the same size or slightly larger than your primary boot drive will do.
Get a self powered and not a Mac port powered drive! Why? Because Mac's can cut off external hardware drawing too much power which can cause issues trying to boot from the clone.
If you have Filevault enabled on your boot drive, then this User Tip has no further use for you, sorry. To de-Filevault your drive will likley require 50% or more free space available to write all the unencrypted bits. Once the drive is de-Filevaulted it can be cloned.
Connect the new drive to the Mac and open Disk Utility in the Utilities folder. (say "no" to TimeMachine, use another drive for that). Click on the external drive on the left and click Erase, move the slider for a 0x-3x secure erase (SSD's not needed), OS X Journaled/GUID as the format and a name like "Mac10.6Clone" (different than the internal drive) and click apply. Go watch a movie. Warning: Formatting a drive or partition erases all data.
If the scrubbing of the drive fails, try again and if it fails, the drive is bad and needs to be exchanged. All new drives have bad sectors, this scrubbing will remove most of them and make for a much more reliable backup. Highly recommended for all hard drives if it hasn't been done before, SSD's no need.
Once the scrubbing is finished check under Partition that Option: GUID and Format: OS X Extended Journaled or make it that. Quit Disk Utility.
Scrubbing is only for hard drives and not needed if it was done once before as the bad bits were previously mapped off. However if shock to the drive occurred or the 0x erase wasn't good enough, or there is unwanted data, then of course repeat scrubbing again. I've found 0x for defeating software recovery techniques, 3x seems to be adequate for mapping off the bad bits. 7x for defeating magnetic recovery techniques. Physically destroying the drive for the utmost sensitive files.
Download Carbon Copy Cloner, it's the best cloning software and the only one that also clones the most vital RecoveryHD partition for restoring OS X fresh from Apple. Later you will use it to update your clone once your on the new internal drive. (Optionally is SuperDuper, but it doesn't do the RecoveryHD.)
Use CCC to clone your present internal drive to the external. Do not use the same name as the internal drive on the external, or change it before you boot from the clone using the Finder.
You don't need to mess with CCC preferences for the first clone as it has to to everything. Later when updating the clone it will save the changes between updates and take up much more drive space. If you want to maintain a pure clone, then set that in CCC preferences before doing a update, this is what I advise as it maintains space for later less you need to install additional software or move files for data recovery etc. while booted from the clone drive if the internal drive fails to boot up.
Repair permissions afterwards
Once the clone is finished, use Disk Utility to repair permissions on both drives. Reboot the Mac and hold the option/alt keys down on the built-in or wired keyboard for OS X's "Startup Manager" there you can select the clone to boot from. Check it out and be careful with your navigation as everything is duplicated, including the pathnames/shortcuts to your internal drive. (Ignore Disk Utility warnings that repeat, those are just changes Apple made)
Internal drive switch
If you have a internal drive switch or install at this time, now is the time to do this. If you run into problems you can option/alt key boot off the clone drive, use the computer like before and get online. TimeMachine doesn't have boot to use ability only boot to restore, which mandates your internal drive be immediately fixed and that's not always very convenient for many.
More advice here:
Reverse cloning, etc
To reverse clone, follow the same procedure as above. First formatting the new or erasing (secure zero or 3x overwrite) the corrupted drive (if so) in Disk Utility first and then using CCC (all while booted from the external clone of course), to reverse clone. CCC should ask to restore the RecoveryHD (for 10.7+) in this first clone process, so check that it will/does.
Bootable clones make ideal data recovery drives, just boot from it and grab the latest copy of files (or install Data Rescue for deleted files/corrupted drives) on the primary boot drive and secure erase and reverse clone perfection back on.
It's advised to maintain several time dated clones each on separate hardware, this way one can revert to a earlier OS X version or use older versions of software and to protect against accidents and other unseen acts. Provides maximum software and hardware protection.
Do NOT reverse clone onto a new or different model of Mac as there are hardware driver differences so OS X is ever so slightly different. (But in some cases it's possible, not going there). Instead to be sure, use Migration Assistant in the Utilities folder and target the clone drive as the source of User Accounts and programs you want to transfer to the other Mac.
SSD advice, TRIM
If you cloned to a third party SSD, you need to enable TRIM support on it and after each OS X update/upgrade using third party software. Check MacTracker.com for the two versions and update it.
Remember 10.8+ users may have to right or option/alt click to "open" software downloaded from the Internet.
TRIM is wear leveling software so that data is written to the least used areas of a SSD because they have limited write capability. Thus SSD's don't have scrubbing ability to securely delete data off of them. Software and forensic equipment is widely available to read all data on a SSD, included deleted data.
SSD's are on many portable tech electronic devices and becoming widespread on computers now. The NSA approved method for data destruction on non-magnetic media (including thumb drives) is to grind them into a fine powder. Since a lot of Apple hardware is coming sealed up, Fusion drives (hard drive + flash memory), and non-user accessible, the entire device will have to be destroyed in order to remove unwanted data from it.
For privacy when reselling without vital data, it might be possible to fill the SSD near completely two times with a LOT of tiny small files to overwrite all areas, but that's no guaranty nor a legal/military/government approved method.
Update your clone
Update your clone occasionally and before a major internal boot drive change using CCC, it will take less time than a full clone each time like Disk Utility unfortunately does. CCC also has scheduling ability to perform the updates automatically.
Do not clone a TimeMachine drive.
Clone to another internal partition
Yes you can clone your boot partition to another partition on the same boot drive. Clone to a external drive first and test for backup as you NEED hardware protection also in case the internal drive or Mac dies or is lost/stolen.
Create another partition in Disk Utility equal to your MacintoshHD partition, format OS X Extended Journaled and use CCC to clone MacintoshHD to MacintoshHD 2. Then Disk Utility >Repair Permissions on both. (ignore warnings that repeat, those are just changes Apple made) Set CCC to maintain a pure clone for this partition as not to fill it up.
If you can't create another same sized partition, you likely need to reduce files and (if a hard drive, SSD's no need) perform a reverse clone while booted from the external clone to shift all OS X data up further on the drive to make room at the bottom for the new partition.
Usefulness in this clone on another partition thing is CCC can update it in the background on a schedule all by itself. You have a resource to recover accidentally deleted files and if your primary OS X MacintoshHD partition is not booting due to software reasons or other software issue, you can immediately option/alt key boot from the cloned partition. A excellent choice for laptop owners away from a fast/reliable Internet connection to fix OS X by redownloading it over itself or carrying a external clone or TM drive around with you. It only takes seconds to boot from the cloned partition.
It's obviously taking half your boot drive space, however it only takes Disk Utility a few seconds to rename/quick reformat (no secure erasing) to make it available for emergency storage needs and later one can use CCC to clone it again in the background. Also on hard drives the second 50% of the drive is slower than the first 50%, so you may realize this while booted from the second partition.
Enjoy peace and software stability.