So, I was asked this the other day, and it can be quite tricky, basically, you need to boot from an installation disk and run Disk Utility to erase the disk.
Now as the MacBook Air doesn’t have FireWire then Target Disk Mode is pretty much out of the question (although this is now an option on Thunderbolt equipped MacBook Airs), so we will have to either try a remote disk option, or boot from USB.
In this guide I will run through the steps required to complete this task using a bootable USB key, but if you wish to go the Remote Disk option then more info can be found on Apples Support Site at http://support.apple.com/kb/HT2129
For this you will need
- An 8GB USB key or Hard Drive (or larger)
- A Retail Operating System Disk
- A Mac with a Superdrive
Ok, so first of all if you have any data on the USB key, make a backup of any data you have on your USB key
On your Superdrive equipped Mac, insert your Operating System Disk & USB key and open Disk Utility
In Disk Utility, click on the OS Disk and then click on the ‘New Image’ icon at the top of the window, pick a name and save it to your Desktop
Once the image is created it will appear in the left hand window of Disk Utility, click on it and then click on ‘Images’ in the menu bar at the top of the screen and choose ‘Scan Image for Restore’
Once that has verified your image, click on your USB key in the left hand window and and choose the ‘Restore’ tab in the main window.
Next drag the image you created to the ‘Source’ field and drag the USB key to the ‘Destination’ field, tick the box that says ‘Erase Destination’ and click on ‘Restore’.
This will erase your USB key and replace it with the contents of your image, so all of the files on the OS disk will be transferred to the USB key, with the end result being a bootable USB key
Once this has completed, eject the USB key and plug it in to your MacBook Air and hold down the ALT key whilst powering it on, you should be presented with the option to boot from the hard drive or the USB key, if you have set a firmware password though you will need to enter this to gain access to the boot menu
When you have booted from the USB key it will vary depending on which OS you have created it from, but usually you will have to agree to a license agreement to get to the pre installation screen, at the top of the screen you should see a Utilities menu, you must click on this and run Disk Utility.
When you are in Disk Utility click on the hard drive you wish to erase, then click on the erase tab, you can then choose which type of deletion you require, I would choose the highest security option, which involves multiple passes of the hard drive, but this can take several days depending on hard drive size, if you don’t have the time then choose a lower security option, but remember more secure = many hours = less chance of data being recovered.