Sometimes you want to make an exact copy of all the files on your Mac–also known as a snapshot copy or a clone. This means a one-time exact duplication of your existing computer (which is different from Time Machine, which makes incremental backups as files on your computer change.)

Some reasons why you might want to do a one-time copy:

• You are about to perform a serious system upgrade or install some pirated software, and want to be able to easily revert to your old system if something horrible happens
• You are about to take your Mac in for repair, and you’re afraid they will wipe the hard drive during the repair
• You are about to go away on a really long trip with your computer, and want to leave a complete copy of your entire hard drive at your home
• You want to have an extra backup in addition to Time Machine, in an offsite location (I do this; I have a clone of my drive that I erase and re-clone every 4-6 months, and I keep it at a friend’s house)

What you will need to create this copy:

• An empty external hard drive. It can be any brand and does not need to be Mac-specific, but must have as much space as the hard drive on your Mac.
• Carbon Copy Cloner software, free but please make a donation to the author.

How to create a snapshot backup that is bootable, so you can get up and running in an instant if your Mac hard drive fails:

1. Attach the drive to your Mac and reformat it for Mac. Follow these instructions on how to reformat for Mac.

2. Download and install the Carbon Copy Cloner software, and run it. A box pops up.

3. On the left-hand side, under Source Disk, click on “Select a source” and choose your computer’s internal hard drive. (It’s usually the silver one.)

4. On the right-hand side, under Target Disk, click on “Select a target” and choose the empty external hard drive that is connected to your Mac. Underneath make sure it says “Backup everything” and you should also get a message saying “This volume will be bootable.”

5. Double-check that you have selected the correct Source Disk and Target Disk. If you’re not careful you can erase your computer hard drive so take your time!

6. Click the Clone button. The copy will probably take a few hours and it’s best if you aren’t using the computer while this is happening. So you might just let it run overnight.

7. When the copy is done, eject the external hard drive and store it in a secure location. That’s it!

The image of the cloned dogs is from jurvetson, on Flickr Creative Commons.