Because OS X Lion is currently only available on the app store, there is no option to do a clean install, only an upgrade from Snow Leopard. Below I will take you through the steps of how to create a bootable OS X Lion DVD or USB drive.
Note: Remember to always backup any important files that are on your computer when making changes to your operating system, just in case!
Step 1: Download OSX Lion from the App Store.
The first thing we need to do is purchase and download OS X Lion from the App Store. Be aware that this is a 4GB download so unless you’re lucky enough to have a fibre optic connection, chances are you may want to go grab a cuppa while it downloads.
When the download completes it will launch the installer. Make sure you exit out of the installer as this will start the update and stop us from creating our bootable DVD.
Step 2: Locate the downloaded OSX Lion app.
Open your applications folder and locate the “Install Mac OS X Lion” programme. We then need to right click on the Installer and select “Show Package Contents”.
Step 3: Copy InstallESD.dmg to your desktop
Open the “Shared Support” folder. You should now see a file called “InstallESD.dmg”. This is the file we will be using to create our bootable DVD. Copy this file to your Desktop or anywhere you have easy access to.
Step 4: Start Disk Utility
We then need to start the Disk Utility program. This is located in your Utilities folder, which is inside your Applications folder. When it launches you will see all of the storage devices in or connected to your computer.
For burning a bootable DVD read step 4a or for creating a bootable USB drive read step 4b.
Step 5a: Insert a Blank DVD
For users that want to create a DVD for installation purposes, you now need to insert a blank DVD into your Mac.
In Disk Utility, click the “Burn” icon at the top and then select the “installESD.dmg” file that we copied to our desktop in step 2.
Step 5b: Insert a blank USB drive
For users that want to create a bootable USB drive for installation purposes, you now need to insert your blank USB drive now. When the USB drive appears in the menu on the left select it and then click the restore tab at the top.
In the source field we need to select the “installESD.dmg” file that we copied to our desktop in step 2.
In the destination field choose the partition of the USB drive we just inserted.
Note: We will be erasing anything that is currently on your USB drive so make sure you backup anything that is on the USB drive that you want to keep.
Click the Restore button at the bottom.
Step 6: Installing OS X Lion
To install OS X Lion form the Bootable DVD or USB we just created you will need to restart your Mac with the DVD or USB connected. As soon as it starts to reboot you need to hold down the option key until it displays a selection of the bootable media available. Select the DVD or USB you created and the installation will start. Follow the onscreen instructions to install a clean copy of OS X Lion.