Since it doesn't come with an optical drive, giving MacBook Air buyers a DVD of the operating system wouldn't be very helpful. That's why Apple's lightest laptop comes with the USB Software Reinstall Drive -- a very small white USB drive. Sometimes USB flash drives are referred to as "Thumb Drives," but this one is more like the size of a baby's finger.

The USB stick that comes with the MacBook Air is the best way -- and in many situations, the only way -- to restore/reinstall your MacBook Air if something goes wrong. If you have the US$100 external USB SuperDrive, then you can try to use a DVD, but I found that my MacBook Air wouldn't even boot from my original Snow Leopard DVD. (I believe this is because the DVD's build of Snow Leopard is a lower version number than the Air originally shipped with, but I am not sure.)

Since my MacBook Air first arrived I have been afraid of losing the USB recovery drive. According to someone on the Apple Discussion Forums, Apple may be willing to provide you with a new one, free of charge, if you lose yours, but what I really wanted was a backup. (I have done the same thing with my Snow Leopard DVD when I bought it, just in case it was lost or damaged.) In this case, I wanted to duplicate it onto another USB drive.

I tried 'cloning' the Reinstall Drive using SuperDuper!, which completed without error, but after it was done, my MacBook Air would not boot with the new USB drive.

Apple created the drive so that when it is mounted by OS X, it appears as a DVD, not a USB drive. This means that it was not available for me to use as a "Restore" source in Disk Utility. I could not find any way to make a copy of the disk image from Disk Utility. (If one exists, I'd be happy to hear about it.)

That's when my GeekInstincts kicked in.

If Apple wanted me to treat it as a DVD drive, that's exactly what I would do. In the past I have backed up my OS X DVDs using the Terminal, and I wondered if the same thing would work here. (Spoiler alert! It did.)

Duplicating a CD or DVD the Unix way

Rather than using a GUI program such as Roxio Toast or Burn, we're going to use the Terminal. Why? Because unlike those two programs, the Unix way is free, simple, and "just works." Plus, you end up with a disk image, which you should be able to use to burn an actual DVD on just about any computer.

The steps are fairly simple:

1. Create an .iso file of the official Reinstall Drive

2. Mount the .iso file in Disk Utility

3. Mount a generic USB drive

4. Use the "restore" function in Disk Utility to copy the .iso file to the generic USB device.

5. Save the .iso file in case you lose the Reinstall Drive and your generic backup.

Note: you can do this same process with any CD/DVD and any Mac. In the past I have done it with Microsoft Office, iWork, and others. This article is addressing the MacBook Air specifically, but the same steps would work equally well for other media.

The Disclaimer

We are going to be using Terminal.app (found in the /Applications/Utilities/ folder in the Finder) to run a few commands. If you are not careful in the Terminal, you can do some serious damage. Then again, the same thing is true about a car. So, look both ways, stop if you aren't sure about something, and (whenever possible) copy & paste commands rather than typing them manually, to avoid typos.

The command we will be using is /bin/dd, which I suggest you think of as "Data Duplication." Wikipedia says that it probably originally meant "Data Description," and it is often jokingly referred to as "data destroyer" or other scary-sounding names.

Read slowly, take your time. There should be no real danger unless you are extremely careless. Don't proceed unless you know what you are doing and have verified your backups.

Step By Step

1) Insert your Reinstall Drive and make sure it appears in the Finder

2) Open Terminal.app (in Finder press Command-Shift-U to quickly go to the Utilities folder)

3) In Terminal, type (or paste) these following commands:

DEVICE=`mount | fgrep 'Mac OS X Install' | awk '{print $1}'` 

 if [ "$DEVICE" != "" ]; then ; echo $DEVICE ; fi 

You should see something like "/dev/disk?s?" where the ?s are replaced by numbers. If all you get a blank line, something went wrong. Make sure the drive appears in Finder.

4) Still in Terminal, type/paste this line:

diskutil unmount "/Volumes/Mac OS X Install" 

which should tell you "Volume Mac OS X Install on disk?s? unmounted" (again, where ? and ? will be numbers). This will unmount but not eject the drive.

5) READ this entire step, and make sure you understand it, before you do anything.

Now we will tell dd to:

a) read input from the "$DEVICE" named above. This is the "Input File" which is identified using if=/dev/disk?s? (where ? are numbers)

b) copy what you read (in step 'a') out to a new file. This is the "Output File" which is identified using of=WhateverYouWant.iso (I went with airinstall.iso for simplicity and clarity).

c) We also need to tell dd to use a Block Size of 2048 (this last part may not be 100% necessary, but I have seen it suggested and it is how I have done mine, and it worked).

If you put all of that together, it should look like this:

/bin/dd if="$DEVICE" of="$HOME/Desktop/airinstall.iso" bs=2048 

WARNING: if, by some bizarre chance, you already have a file named "$HOME/Desktop/airinstall.iso" be sure to move or rename it before you enter that line. Otherwise it will be overwritten.

(The Output File does not have to be saved to the Desktop, I just chose that because it is a place most people will notice.)

If you see an error "dd: /dev/disk?s?: Resource busy" then the device did not unmount properly in step #4.

If you copied my "DEVICE=" line above in Step #3, you should be able to use "$DEVICE" in the 'dd' line to automatically fill in the proper device.

Note!

• the 'dd' command may run for 20–30 minutes, or longer. Don't panic. Just go do something else for awhile.

• nothing new will appear on the screen until 'dd' is finished.

• When it is done you should see something like this:

  3738954 0 records in
  3738954 0 records out

The "records in" should equal the "records out" (although your number might not be the same as mine).

6) Assuming everything went as expected, you can now tell the computer to eject the Apple Reinstall Drive by entering this line in Terminal:

diskutil eject "$DEVICE" 

It should tell you that the device was ejected. If so, it will be safe to physically disconnect the Apple Reinstall Drive from the USB port of your computer.

7) Open the .iso file in Disk Utility. If you used my /bin/dd line above, you can now enter:

open -a "Disk Utility" "$HOME/Desktop/airinstall.iso" 

and Disk Utility will open and the .iso will appear in the left sidebar.

Note: we are now done with Terminal.app. You may quit it and switch over to Disk Utility for the next steps.

8) You should see airinstall.iso in the left side of Disk Utility window. If you look at the bottom of the window you will see it is not mounted. Click the "Open" button on the top toolbar to mount the .iso file.

NOTE: When you mount the .iso file, Finder will probably jump up and show you the window like an excited schoolchild. If that happens, just switch back to Disk Utility.

This is what Disk Utility will look like after you mount the .iso:

Note that the capacity it shown, and the open button is now a greyed out "Mount" button, and the "Eject" button is now available.

9) Insert your generic USB drive now. You will need an 8GB USB drive. I'm usng a SanDisk because it's what I already had.

Here is Disk Utility showing my USB drive. A few important things to notice:

Make sure the drive is formatted as "Mac OS Extended" or "Mac OS Extended (Journaled)."

The "Name" of your drive isn't important. Notice mine is "USB_MOUNT" but the important thing is that I have it selected in Disk Utility.

10) We are now going to tell Disk Utility to "Restore" the .iso's "Mac OS X Install" to "USB_MOUNT" by doing three things:

a) click the "Restore" button (see red box below)

b) Control-Click ("right click") on the "Mac OS X Install" line, and select "Set as source" as shown here. You could also just drag the Mac OS X Install volume over to the Source: field in the right-hand pane.

c) Control-Click on the "Name" of your generic USB drive (mine is "USB_MOUNT") and choose "Set as destination." Again, you could alternatively drag-and-drop the volume icon into the blank Destination field on the right side, per the onscreen instructions.

11) When you are ready to restore, it should look something like this:

If everything looks correct, click the "Restore" button on the bottom right. Disk Utility will give you a "human readable" explanation of what it is about to do:

Read through it to make sure that you haven't accidentally swapped the Source and Destination fields. When you click "Erase" OS X will prompt you for your administrator password. Once you enter it, the restore process will begin.

This took 30 minutes on my MacBook Air, so it's a good time to take that Apple Software Reinstall Drive and put it somewhere safe. (I highly recommend putting it back in the box your MacBook Air came in. You kept the box, right?)

Once the Restore Process completes, Finder will most likely mount the generic USB drive.

Disk Utility will look like this:

Now we're ready to test it, so quit all of your applications, logout, and shutdown the computer.

The Proof is in the Booting

The only real test that matters is whether or not you can boot your MacBook Air with the generic USB drive.

After the computer id turned off, make sure that:

• the official Apple Softwware Reinstall Drive is not connected to your MacBook Air

• the generic USB drive is connected to the MacBook Air

Then power the computer on and press the Option/Alt key.

In a few moments you should a screen offering the option of booting from your hard drive or the USB drive. Choose the USB drive.

When it finishes booting, it will start into the "Install OS X" screens and ask you to select a language. Don't panic. Select a language, and then at the next screen you can exit out of the installer.

(If you want to set an Open Firmware Password, this would be a good time to do it, since you can't set it when booting off the internal drive, but if you do, remember that if you forget the password, a trip to the Apple Store is your only hope of recovering it.)

That's it

The USB drive won't work on any other computer, but it can add a little peace of mind to know that you have another copy of your restore drive in case you ever need it.